BLACK RIVER LIMESTONE. 



39 



observations noted on Black river, and not having a common collection of specimens to resort 

 to in order to understand each other. The term Mohawk was applied by Mr. C. to the inter- 

 mediate mass, to which no name in this report is given but the Base of the Trenton limestone. 

 This fact is one of many, to show the superiority of names given to rocks, taken from localities 

 where but one rock or mass exists. The standard ever exists — it varies not ; and its charac- 

 ters, associates and position can be ascertained by any one at the place after which it is named. 

 The Birdseye limestone on the Mohawk is readily distinguished from the other rocks by its 

 light dove-color, which, by long exposure to the weather, becomes of a light ash grey or white. 

 It is usually in thick layers, which are straight, having very little interposed matter between 

 them ; with vertical joints which are so straight and even as to give to the rock, where quar- 

 ried, the appearance of a wall. In its grain, it is very compact ; fracture smooth, and from 

 being brittle, is an easy rock to work. 



The above wood-cut, from a drawing by Mr. Emmons, exhibits the Fucoides demissus, the 

 characteristic fossil of the Birdseye limestone, as it appeared in the quarry at Fort-Plain. It 

 is a faithful representation, upon a reduced scale, of a vertical section of a part of a layer of 

 the limestone. 



As the fossils of the State form a distinct department of the survey, no more illustration 

 could be given in this report, than sufficed to show the character which these bodies give to 

 the rock or group to which they belong. 



The Fucoides demissus is found in this rock in every locality on the Mohawk. It is there 

 very distinct, as in the wood-cut ; the plant having been removed, and the cavities which it 

 occupied enlarged and filled with green and black shale, which strongly contrast with the 

 light dove-color of the rock. There and elsewhere, some of the fucoids are also replaced by 

 lamellar carbonate of lime ; the spaces occupied are white, and small in comparison with 

 those of the former kind ; and when few in number, are not so obvious. This is the case 

 with the limestone along Black river. 



