mLLiAMs.] WILLIAMS. 



129 



The consecutive faunas were alsp examined particularly as to condi- 

 tions of environment with which they were originally associated, as indi- 

 cated by the lithologic character of the deposits. The scope of the 

 work may be indicated by the following quotations : 



The aggregation of species into faunas, the blending of one fauna with another, the 

 rarity or abundance of particular species, variation in form or size or modification of 

 specific characters, the extinction of old and the initiation of new forms— all these 

 become the most delicate tests of change in the physical conditions, the record of 

 which constitutes the geological history of the earth. 



For the correct solution of this problem the laws of geographical distribution form 

 as important an element as geological sequence. The attempt to apply such principles 

 to the study of the Devonian and Subcarboniferous deposits is no simple task, but 

 the very fact that their faunas offer so great variation and difference in their combi- 

 nations makes this series particularly attractive for the purpose. 1 



******* 



These facts make it plain that over any particular area the faunas shifted back and 

 forth with the advance of geological time. Hence I was led to the simple conception 

 of a fauna as continuing on intact as long as the favorable conditions for its life con- 

 tinued, as shifting its habitat with the elevation or .depression of the land, with the 

 advance or retrocession of the coast line. In such shifting and change of condi- 

 tions, one species after another may drop out and become extinct ; others may suffer 

 varietal modification, and, what is still more important, the sudden appearance of 

 new forms may take place in the midst of the normal fauna— forms new to the local- 

 ity only, or entirely new, so far as our knowledge of the fossils can tell us. Merely 

 from the initiation of the new forms in the fauna we can gain no clew of its origin 

 but the study of its relations to allied forms of other faunas may enable us to decide 

 whether it is a modification of some older form or the forerunner of a new type, 

 marking a later geological stage. 2 



The following is a summary of the order and general relative position of the faunas 

 from the Genesee slate to the Barclay coal, which my present knowledge leads me to 

 believe is true for the meridian passing through Ithaca, New York, running south- 

 ward. 



(1) Genesee slate fauna, 



(2) Portage group fauna, distributed through approximately 1,300 feet of strata, 

 but interrupted by the intrusion of the Ithaca faunas and several sub- faunas. 



(3) Chemung fauna, occupying at least 1,200 feet of strata, with perhaps two sub- 

 faunas, and driven out or destroyed by the presence of the conditions marked by the 

 deposit of red and gray Catskill rocks. 



Within the limits assigned to the Portage group in the western part of New York 

 State, I believe should be included for this meridian all those deposits lying between 

 the Genesee shale and the lowest yellow-brown shale and sandstones which carry the 

 true Chemung group fauna. 



This series, as a whole, may be described as arenaceous, dark-colored shales with 

 the Cardiola speciosa fauna, toward the top running into wave-marked, tough, arena- 

 ceous deposits, almost totally barren, so far as known. 



The passage between this series and the true Chemung is stratigraphically indis- 

 tinct, but in a general way it may be recognized by the clearer separation of the 

 argillaceous from the arenaceous deposits after passing the line, and the appearance 

 of lighter-colored sandstones in the midst of softer argillaceous shales, in which iron 

 nodules and iron stains become more conspicuous than below. 



The shales of the Portage below are thinner and of more greenish tint, and its 



1 U. S. Geol. Surv., Bull. No. 3, p. 6. 2 Ibid., p. 8. 



Bull. 80 9 



