BLACK MARBLE OF ISLE LA MOTTE. 



Ill 



county, tlic banks of the Black river arc formed of the same rock. It is known by quarry- 

 men as the Secen-fcot Her. But here, where it is exposed in the banks, it is lumpy, and 

 appears unsuitable for marble, or it does not seem to be sufficiently homogeneous to constitute 

 a valuable material except for ordinary purposes. At Watertown its position is clearly seen, 

 being between the Birdseye and Trenton limestone. Some discrepancies have ajipcarcd in 

 the annual reports, in relation to this mass ; they have arisen in consequence of mistaking the 

 Chazy limestone for the one under consideration. 



This rock has been described as thick-bedded, and rather massive, with obscure planes of 

 separation. Those planes, when separated, are not smooth, as if a partial desiccation or dry- 

 ing took place prior to the depositicfn of the next layer ; but they are quite regularly toothed, 

 or the planes are studded over with projections, many of which are quite sharp and pointed. 

 The diagram No. 39 will explain what I mean, better than words can express : 



39. 



These projecting points are composed of fibrous crystals of sulphate of stronlian, from one 

 half to an incii in length. The occurrence of the strontian in this connection, illustrates the 

 manner in which these externally fibrous masses are formed in the Niagara and other lime- 

 stones ; only in the former, the fibrous crystals were soluble, and hence have disappeared, 

 leaving only their impress ; while in the latter, the crystals are insoluble, and remain. I have 

 observed the same structure in all the limestones : for its occurrence, it requires a rapid 

 accumulation of materials, and which remain for some time in a soft plastic state. 



The fossils of this rock, though not very numerous, are yet worthy of notice. At Water- 

 town, Glen's Falls and Chazy, the Columnaria sulcata is quite abundant, sometimes in masses 

 of the size of half a bushel. Large orthoceratites are also common, some- of which are ten 

 feet long, and twelve inches in diameter ; they are common also in the birdseye. Another 

 fossil, allied to this genus, is formed apparently of many cones placed within each other, 

 enlarging in the direction of the mouth ; or in other words, the fossil is a series of cones 

 within each other. 



It is not very clear how such masses as this should be treated ; as I remarked in the first 

 paragraph, it is unimportant simply as a rock, and becomes so only in consequence of the 

 valuable material of which it is composed. Were it not for this, it might be placed with the 

 Trenton or Birdseye ; yet according to the principles of modern geology, if carried to their 

 full extent, it requires to be separated from both, on account of its lithological characters, and 

 the distinct kinds of fossils it contains : scarcely one belonging to this mass is found to have 

 passed into the Trenton limestone. It is a great evil to multiply unnecessarily the number of 

 rocks ; and I am sensible that unless a mass has a greater thickness than this, it ouglit gene- 

 rally to be merged in that to which- it bears the greatest similitude. 



