April 14, 1905.] 



SCIENCE. 



585 



existing coasts is all that is necessary to prove 

 how absurd is the idea that the ' formation ' 

 of a given area has any necessary connection 

 with the horizon or time-place in the geolog- 

 ical column indicated by a given fauna. The 

 two categories are, in a broad sense, incompat- 

 ible, one indicating merely local physical and 

 dynamic conditions, and the other the stage 

 of evolution of the organic assembly inhabit- 

 ing an area probably with entirely different 

 boundaries. 



Mr. E. O. Ulrich expressed his belief that 

 the idea that faunas required a long time to 

 migrate from place to place is an unjustifiable 

 assumption. The migration of the Paleozoic 

 faunas, which included mostly shallow water 

 organisms, was limited to zones adjacent to 

 the shore line. The Paleozoic continent had 

 much less relief than the present surface and 

 the adjacent water basins were shallow, thus 

 favoring rapid migration. With slight relief 

 a small amount of tilting would cause rapid 

 submergence of large areas with immediate 

 migration of marine life. Therefore, it would 

 result that like fossil faunas indicate at least 

 essential contemporaneity. The case cited as 

 illustrating such geologic conditions with the 

 rapid spread of a fauna was that afforded by 

 the western and southern formations of the 

 Richmond group in the uppermost Ordovieian. 



Mr. Ulrich stated that his working hypoth- 

 esis is that one slowly modifying fauna ex- 

 isted continuously on the outer border of the 

 continent while another occupied, on the whole, 

 much shallower and frequently changing 

 basins upon the surface of the continent. On 

 account of the comparatively unstable condi- 

 tions prevailing there the epicontinental fauna 

 was subjected to many vicissitudes not shared 

 by the outer fauna. Hence considerable and 

 often very great modifications of its character, 

 both local and widespread, took place much 

 more frequently than in the outer fauna. 

 When conditions were favorable, faunas of 

 the inner basins were, in some cases, re- 

 plenished, or in others, perhaps only slightly 

 modified by accessions from the outer faunas. 



Special emphasis was put by David White, 

 the next speaker, on the relative insignificance 

 of the time, as measured by sedimentation, re- 



quired by those faunal migrations which are 

 not marked by changes in the composition of 

 the fauna and recognizable mutations of the 

 species. The interval between the two great 

 Pleistocene ice invasions was ample for the 

 migration of the flora and even the formation 

 of peats in the thin deposits of interglacial 

 clays. Between the retreat of the last ice 

 sheet and the restoration of the faunal and 

 floral equilibrium, as we now find it, the inter- 

 val, as measured in sediments, is geologically 

 not macroscopic. The practical contem- 

 poraneity, in the geological sense, of an iden- 

 tical fauna in the various parts of its distribu- 

 tional province is shown by the general agree- 

 ment and harmony in these parts between the 

 marine invertebrates and the other contem- 

 poraneously characteristic classes of organ- 

 isms, including marine vertebrates, land plants 

 and land vertebrates, whose directions and 

 routes of distribution varied "widely. A plea 

 was added for a closer study and a more 

 scrutinous characterization of species, taking 

 into account not only the contemporaneous 

 variation of the organism, but also especially 

 the variations or mutations occurring within 

 the duration of the specific type, some of these 

 mutations being of the most restricted vertical 

 range, and consequently of greatest strati- 

 graphic value. George Otis Smith, 



Secretary. 



DISCUSSION AND CORRESPONDENCE. 



THE WESTERN SIERRA MADRE MOUNTAINS. 



To THE Editor of Science: The geograph- 

 ical and geological expedition organized by 

 Col. W. C. Greene for the study of the western 

 Sierra Madre Mountains of Mexico has ac- 

 complished half of the journey proposed. The 

 party, consisting of Professor Robert T. Hill, 

 Messrs. John Seward and F. H. Fayant and 

 the writer, which left New York February 4, 

 was .delayed on its journey to El Paso by 

 blizzards in Canada and the central states and 

 exceptional cold weather in Texas. News of 

 heavy snowfall in the mountains caused 

 farther delay in El Paso, which was utilized 

 by the party for a run across the arid region 

 along the Mexican boundary as far as Naeo, 

 Arizona, and thence to Cananea, Sonora. The 



