266 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



tries. In New York this kind of sandstone occurs at the very dawn 

 of life, or after considerable disturbances, or it is impregnated with 

 magnesia or salt. We remark also that only two fossils occur in 

 micaceo-siliceous sandstone here ; while sixty at least (principally 

 Dimyaria and Gasteropoda) occur in that rock in Wales, where we 

 meet with micaceous and hardened strata in far greater abundance 

 than in New York. The frequent occurrence of volcanic overflows 

 in Wales might prepare us for this, such events possibly producing 

 the mica by metamorphosis ; at all events it is supposed that mica 

 was formed more or less after deposition. The arenaceous limestone, 

 poor in middle North America, abounds in evidences of life in the 

 Bala group of Wales, which corresponds with the Trenton group of 

 New York. Table YIII. shows also the remarkable and unexpected 

 fact, as elicited by the careful investigations of Mr. Salter, that simple 

 limestone contains little more than 2 per cent, of all the fossil ap- 

 pearances in Wales, — New York, after diligent search, yielding 14 

 per cent. This is contrary to the supposed favourable properties of 

 the carbonate of lime ; but it in reality depends on the comparative 

 powerlessness of any single substance to maintain life. 



We see in the same Table, that Welsh mudstone or pure clay con- 

 tains 1 1 per cent, or nearly, by which it would seem that there is 

 the occasional presence of lime. With further examination, most 

 probably the 14 j)er cent, of life-evidence in pure limestone, as re- 

 gards New York, will be sensibly lessened. 



Tables V. and YI., which distribute the Silurian fossils of our 

 two areas into their several sediments, are not without consi- 

 derable interest. They are in fact palaeozoic " dredging-tables." 

 We see the fossils of both usually affecting the same sediments, and 

 in tolerably proportionate numbers — if few in New York, then few 

 in Whales ; and contrary^dse, while some species are seen only by 

 ones and twos in certain strata, others {Crustacea, &c.) fill up nearly 

 every group. The depths (shallow, middle, or great) assumed in the 

 one basin are observed in the other, with occasional deviations on 

 account of local causes. The Plantce are inserted provisionally. 

 The ZoopJiyta, Crinoidea, and Cystidea are seldom or never in 

 shallows ; for most of them require still water. 



e. Divergence. — We have now to consider briefly a subject which, 

 perhaps, has not been sufiiciently attended to, although curious in 

 itseK and not without several important bearings — upon geographic 

 distribution, for example, and on the recurrence or vertical range of 

 animal life. It is this : a very large proportion of the Silurian fauna 

 is not constant to one form of sediment, but deviates into others, 

 and these not necessarily on the same horizon. It is noticed by 

 Alcide D'Orbigny* in the following words : — " The same fossil fauna 

 is found in the same geological horizons in beds of entirely different 

 mineral character." This group of organic remains may be called 

 " divergents," — not using the better term " aberrants," because it 

 has been already appropriated. 



By the New York Table of Possil Habitats (Table V.), we find that 

 ^ Elem. Paleont. vol. 1. p. xxxiv. 



