28 GEOLOGY OF THE SECOND DISTRICT. 



observed among scientific men, of adhering to that name which has priority ; hypersthene 

 rock, being the name apphed to the same mass in Scotland by McCulloch, previous to its 

 discovery in New-York. 



This rock is very intimately related to granite ; though its composition is quite different, 

 yet its general aspect is much the same, being unstratified, and traversed by divisional planes 

 in a manner corresponding to the several varieties of the rock. It is, however, darker, espe- 

 cially on a recent fracture ; but when it has been exposed to the weather on the sides of 

 mountains, it appears at a distance like the common grey granite, its light color being due 

 exclusively to the influence of atmospheric agents. 



The predominant color of this rock is a smoke grey, either light or dark. The color most 

 constantly associated with this is a dingy green, which belongs to the labradorite, and not to 

 the odier minerals which happen to be present. 



The pieces of rock obtained near the surface are traversed by natural joints, which also 

 belong to tlie labradorite ; they appear in lines of a much lighter color, but more or less 

 broad, in consequence of which it is often checked or spotted. Wlien masses affected in this 

 way are polished, they appear mottled. 



By exposure to the atmosphere, the rock becomes light, or ash grey ; its surface is often 

 friable from decomposition, and in this respect it is much like the ordinary granite. Espe- 

 cially does this change or action of the atmospheric agents take place on the exposed sum- 

 mits of the mountains, where large blocks crumble down rapidly, and their debris is carried 

 to the valleys by rams and toiTcnts. It even appears quite probable that those large blocks 

 at the summit of Adirondack mountain are nearly in situ, having been as it were quarried 

 out by the effective agency of water, air and frosts. However this may be, it is evident that 

 decomposition is far more rapid and powerful in those elevated regions than iipon the plains. 

 In the course of time, therefore, no inconsiderable change may occur in the height of a moun- 

 tain ; the contour or shape of the summit may appear nmch the same, while its actual eleva- 

 tion is slowly but constantly diminishing. 



Hypcrsthenc rock may be described under the following varieties, though I would here 

 remark that I attach very little importance to them, as they can rarely be made out without a 

 close and careful inspection : 



1. The most common of the varieties is composed wholly of labradorite, though to tlie eye it appears like a 

 mixed or compound mass, as it has the aspect of being made up of two distinct minerals. 



2. Labradorite and hornblende : the hornblende appears usually to take the place of the hypersthene, though 

 not always ; for sometimes the latter is still present. 



3. Labradorite, hornblende and epidote ; the latter, however, never is in sufficient quanlity to give character to 

 the rock. 



4. Granular labradorite and mica, a variety which is quite dark, and has much the aspect of trap. 



Of the varieties which have been designated, the first, as I have stated, is composed entirely 

 of labradorite. It appears, however, in the rock like two distinct minerals, in consequence 

 of the different states the labradorite is in. Thus, in one state, it is crystallized in particles 

 or masses of sufiicicnt size to admit of cleavage, and even sometimes the size of a peck 

 measure, "but more frequently varying from the size of a pea to a walnut. The other form 



