316 GEOLOGY OF THE SECOND DISTRICT. 



I find it important to notice still further some diversities in the chazy limestone, as we may 

 find them in the ridges here referred to. 



I. Besides the concretionary masses, there is quite a tliick bed of grey oolite; the concretions arc quite small and uniform 

 in size, not exceeding a pin's head. It is clearly within the limits of this rock, as we fmd several feet of the black 

 irrcular-bedded hmcstone, containing the maclurea, which has already been described as one of the characteristic 

 fossils. 



"2. Next above the oohte, are sever.il feet of broken encrinital columns, apparently of the same species as those below. 



3. A hard dark-colored mass follows, containing orthoceratites and maclurea. This is only six feet thick. 



4. Twenty feet above the latter, there is a mass of shaly limestone not less than fifteen feet thick, which retains also the 



fossils of this rock. 



To these strata succeeds a drab-colored mass, which probably belongs to the birdseye. 



The fossils of the chazy limestone have been in part enumerated. The Maclurea is the 

 most abundant, and probably there are several species. Some individuals attain quite a large 

 size ; a fragment of one in the State Collection is seven inches in diameter, and is nearly three 

 inches thick through the centre. This fossil has the general character of the Euomphalus. 

 A much smaller species is common at Chazy, in which the thickness is in a greater proportion 

 to the disc ; it is only about one inch over the flat disc, while its greatest thickness (which is 

 at the mouth, and hence marginal) is half an inch. 



In the mass immediately above the oolite, numerous orbiculs appear. These are quite deep, 

 or rather conical, and we find only the convex valve. This is the lowest situation in which this 

 gemis has yet been discovered : and it is a curious fact, though an analogous one has been 

 stated, that it is a recent genus, and has survived all the changes from this early period down 

 to the present, and probably the number of genera which have survived will yet be increased. 

 There are no inducements for deceiving ourselves in this matter . the great lapse of time does 

 not militate against the existence of genera, though we may conceive it would against that of 

 species ; neither does the long continuance of a genus shorten, in our mind, the days of the 

 earth's age. So far, however, as observation extends, the long-lived genera are confined to 

 small testacea. 



A remark which may not be inappropriate here, is important to be borne in mind, viz. that 

 the lithological characters of a rock control, to a certain extent, the organic forms, without 

 regard to position ; and I adduce in support of it, the fact that most of the forms in the calcife- 

 rous and the lowest limestones, are those found in tlie trenton hmestone. The Isotelus canalis 

 resembles the I. gigas ; and so also with the bivalves, so far as they go, the Orthis and Lep- 

 taena in particular. But an illustration of tliis observation may be had when we compare the 

 lorainc shales with those in the Hamilton group. Many of the forms in the former appear 

 at first view but repetitions of those in the latter. The cypricardites, for example, in each 

 rock, arc remarkable for their close resemblances to each other, although they are so distant 

 in time. This fact, however, is one for which we may give a satisfactory rationale. It is 

 only in the general form that this similarity is found : the species are clearly distinct. 



The chazy limestone at Chazy is about one hundred and tliirty feet thick, but probably it 

 is in much greater force here than at any other point along the Champlain. It is proper to 



