Over the years a number of my students have de- fined stream discharge as the rate at which water leaves the mouth of a river. That is not at all unrea- sonable, since one of the common meanings of "dis- charge" in ordinary language involves unloading or emptying the contents of something, but such stu- dents have missed the important idea that discharge is a concept applicable at any point along the course of a stream at any point in time. Is that the fault of the students or of the textbook they were using?

The discharge of a stream is the amount of water flowing past a certain point in a given unit of time. This is usually measured in cubic meters per second or cubic feet per second.

Table 10.1 lists the world's largest rivers in terms of discharge. The largest river in North America, the Mississippi, discharges an average of 17,300 cubic meters per second. Although this is a huge quantity of water, the Amazon discharges 12 times more water than the Mississippi (Tarbuck and Lutgens, 1996, p. 218).

[I have omitted from this passage an equation for discharge in which channel dimensions are incor- rectly used instead of stream dimensions. That was an issue in my column of March, 1997, but is not the issue here.l I have no quarrel with the statements I have just quoted. The only problem is the placement of the ideas. To immediately follow a general defini- tion of stream discharge with statements about how much water entire river basins discharge to the sea leads students away from the generality of the origi- nal definition.

Thompson and Turk (1993, p. 408), Chernikoff and Venkatakrishnan (1995, p. 391), and Monroe and Wicander (1998, p. 417) have introduced the concept of stream discharge in similar ways. In each case, a definition of discharge that is applicable to the water flow at any point along the course of a stream is fol- lowed by a statement about the discharge of the Mis- sissippi River that implies that there is only one (time-averaged) value for the entire river.

Another situation in which an unfortunate juxta- position of two different truths has led my students away from an essential idea involves faunal succession.

Based on Smith's classic observations and the find- ings of many geologists who followed, one of the most important and basic principles in historical geology was formulated: fossil organisms succeed

one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. This has come to be known as the principle of faunal succession. In other words, when fossils are arranged according to their age, they do not present a random or haphazard picture. To the contrary, fossils show progressive change from simple to complex and document the evolution of life through time (Tarbuck and Lutgens, 1996, p. 178).

That fossils show a progressive change from simple to complex and document the evolution of life through time is a conclusion drawn from the observed succes- sion. The presentation of that idea immediately fol- lowing the statement of the principle has led some of my students to a wrong idea - that the principle of faunal succession is that sedimentary layers may be placed in order of age according to the complexity of the organisms represented by the fossils in the rock. Other textbooks lead in the same direction, for ex- ample,

Knowledge of a fossil organism's place on the evo- lutionary time scale can help date a rock unit in which it is found. The principle of faunal suc- cession states that the animals on Earth have changed in a definite order through time: The ear- liest life forms were simpler than those that evolved later, ... (Chernikoff and Venkatakrishnan, 1995, p. 213).

On the basis of his observations, Smith proposed the principle of faunal succession: that each formation contains a unique assemblage of fossils and that the fossil assemblages succeed one an- other in orderly and predictable fashion (Dolgoff, 1996, p. 271).

These quotations have each been taken out of a con- text in which the importance of stratigraphic super- position was mentioned, so I am not accusing these authors of misunderstanding the stratigraphic basis for the principle of faunal succession. I am accusing them only of presenting the principle in a way that is likely to mislead.

Origins of the Problems

Only in the latter half of this century has stream discharge become a concept dealt with in most introduc- tory geology textbooks. In the books I have examined,

the earliest passage similar to the first one quoted above is:

The volume of water flowing through a cross sec- tion of the stream channel is its discharge. Stream discharge is measured in volume per unit of time (cubic meters per second, abbreviated m3/sec). A small creek may discharge only 5 m3/sec; a small river perhaps 50 m3/sec. The largest rivers discharge hundreds of thousands of cubic meters per second. The mean discharge of the Mississippi River at New Orleans, Louisi- ana is 81,000 m3/sec ... at Obidos, Brazil, some 650 km upstream from the river's mouth, the Amazon discharges about 1.5 million m3/sec (Sanders, Anderson, and Carola, 1976, p. 270).

This passage may be the forerunner of problematic passages in current textbooks, but these authors specified the places on the large rivers where the dis- charge was determined, so they maintained the gen- erality of the concept of stream discharge. (They did have a serious problem with the values given for those places, which are both far too large, but that is another story.)

It had been my intention to track down the origin of the problematic treatments of the principle of faunal succession, but that has proved to be an unrealistic goal. William Smith's concept of faunal succession may have been based entirely on the stratigraphic succession, but in the minds and the writings of others, from his time onward, the concept of progres- sive change toward more modern life forms has been closely intertwined with the idea of faunal succes- sion. For example,

Fossils. This criterion for determining the chrono- logical order of strata takes direct hold upon time, and, therefore, is sure and sufficient. The life of the globe has changed with the progress of time. Each epoch has had its peculiar species. Moreover, the succession of life has followed a grand law of progress, involving under a single system a closer and closer approximation in the species, as time moved on, to those which now exist (Dana, 1863, p. 115).

Readers who focus on this passage without critical attention to its context might well be left with the impression that Dana believed the progress (in the sense of change for the better) of life had been essen- tial to the use of fossils for determining the chrono- logical order of strata. The context shows that Dana understood the stratigraphic basis for knowledge of the faunal succession and that he viewed that suc- cession as showing a progressive change from "lower grades of species" toward "the creation of Man and Mind as the last and loftiest of these culminations" (Dana, 1863, p. 126).

Words of Wisdom from the Past

Some eminent geologists of a century ago empha- sized the stratigraphic basis for our knowledge of faunal succession. Modern writers should keep in

mind how important that emphasis can be to young readers who have in their lives heard a lot more about the theory of evolution than about the princi- ple of stratigraphic superposition.

The solution of this problem [the unreliability of mineral characters for correlation over long dis- tances] was found, and was worked out for the Secondary rocks of England, by William Smith at the end of the eighteenth century. It is sup- plied by organic remains, and depends upon the law that the order of succession of plants and animals has been similar all over the world. Ac- cording to the order of superposition, the fossils found in any deposit must be older than those in the deposit above, and younger than those in that below. This order, however, must be first accurately determined by a study of the actual stratigraphy of the formations; for, so far as regards organic structure or affinities, there may be no discover- able reason why a particular species should pre- cede or follow another (Geikie, 1903, p. 835-6).

Sequence of faunas based on stratigraphy. It is not to be understood that rocks which contain such faunas are classed together simply because they contain certain fossils. This is not the prin- ciple, or at least not the only or the fundamental principle, which causes them to be grouped to- gether. The order of sequence of faunas is first determined by the superposition of the strata. ... in other words, the basis for correlation by means of fossils must necessarily be stratigraphic (Chamberlin and Salisbury, 1905, p. 242).

Discussion

Readers may be misled when two different but re- lated ideas are juxtaposed in such a way that the second idea leads them away from the true meaning of the first idea. Ifhave described two different cases where some of my students have been so led to wrong ideas. One might blame the students for not having thought critically about the ideas. On the other hand, writers and teachers have a responsibility to think critically about how students may respond to the order in which ideas are presented.

I think it is particularly important for writers and teachers to give careful attention to the ideas associ- ated with the faunal succession. Tarbuck and Lutgens (1996) should not have written that the principle of faunal succession is one of the most basic principles in historical geology, for it depends on the more fun- damental principle of superposition. Chernikoff and Venkatakrishnan (1995) should have avoided word- ing that could lead readers to think the principle of faunal succession "states" that the earliest life forms were simpler than those that evolved later. Dolgoff (1996) should not have substituted "orderly and pre- dictable" for "definite and determinable" as descriptors of the faunal succession. (To put my preaching in per- spective, I must admit that it is only late in my career that I came to realize how important it is to emphasize to students that the principle of faunal succession is not based on the theory of evolution - that it is built

on the foundation of superposition. I should have started much earlier to read from the works of old masters such as Geikie, Chamberlin, and Salisbury.) In preparing to write this column, I have come to un- derstand that the story of faunal succession is much more complex than I had earlier realized. The con- cept has a rich history, and it is still developing as new methods of correlation allow us to learn more precisely how organisms were distributed in time the world over. I recognize that use of the faunal succes- sion in geology requires careful consideration of bio- logical concepts as well as of the observed successions of fossils. Since I have never studied paleontology formally (or in any depth informally), I am not the right person to propose how the principle of faunal succession should best be presented to students be- ginning their study of geology at the threshold of the new century. I would welcome a paper by someone who is willing to address the issue more fully than I have here.

References Cited

Chamberlin, T.C., and Salisbury, R.D., 1905, Geology, in three volumes, Vol. II, Earth history, Genesis-Paleozoic

(second edition, revised): New York, Henry Holt and Company, 692 p.

Chernikoff, S., and Venkatakrishnan, R., 1995, Geology: An introduction to physical geology: New York, Worth Publishers, 593 p.

Dana, J.D., 1863, Manual of geology: Treating of the prin- ciples of the science with special reference to American geological history, for the use of colleges, academies, and schools of science: Philadelphia, Theodore Bliss & Company, 798 p.

Dolgoff, A., 1996, Physical geology: Lexington, Massachu- setts, D.C. Heath and Company, 628 p.

Geikie, A., 1903, Text-book of geology (fourth edition, revised and enlarged, reprinted 1923): London, Macmillan and Co., Limited, 1472 p.

Monroe, J.S., and Wicander, R., 1998, Physical geology: Exploring the earth (third edition): Belmont, California, Wadsworth Publishing Company, 646 p.

Sanders, J.E., Anderson, A.H., and Carola, R., 1976, Physical geology: New York, Harpers College Press, 584 p.

Tarbuck, E.J., and Lutgens, F.K., 1996, Earth: An intro- duction to physical geology (fifth edition): Upper Saddle River, Prentice Hall, 605 p.

Thompson, G.R., and Turk, J., 1993, Modern physical geology (updated version): Philadelphia, Saunders College Pub- lishing, 608 p.

3rd International Conference on Geoscience Education

Anyone interested in or devoted to geoscience edu- cation is invited to participate in the 3rd International Conference on Geoscience Education, which will be held at the University of New South Wales in Sydney, Australia, from Sunday, January 16, to Friday, Janu- ary 21, 2000.

The central theme for GeoSciEd III (Dedicated to Teaching and Learning) highlights the concept that, while we may be teaching, we can all learn from each others' experiences and practices. The conference will feature workshops for and by both secondary and col- lege teachers, field trips both before and during the conference (to sites including the Great Barrier Reef and Uluru, that is, Ayers Rock), a social program fea- turing an authentic Australian barbie, entertainment, and a cruise of Sydney harbor, as well as programs for guests.

For further information and/or a copy of the official Registration Circular, contact John R. Carpenter at the University of South Carolina or M. Frank Watt Ireton at the American Geophysical Union. The dead- line for reduced registration fees and for submission

of abstracts is May 31,1999. Additional information can be found at ttp://www.agso.gov.au/geoscied/#feature.

Geology Central Website

A new website called "Geology Central" can be ac- cessed at: http://welcome.to/geologycentral. The site is a database of over 400 virtual geology field trips and has hundreds of links to other geological web- sites. One of the categories with a very large number of links is the national parks section.

Geology Central was developed by two lower- division students at Santa Monica College who have done outstanding work in creating the site. If particular field trips or links that anyone knows of are not listed, we would appreciate having their URLs e-mailed to the address given below. We urge site creators to keep their sites active since we do not want to drop them from our database. We hope that our site will be widely useful to geoscience educators and invite participation and suggestions for improvement.

Richard Robinson

Geology Department

Santa Monica College robinson_richard@smc.edu