106 MINERALS OF MARYLAND. 



Stone, and so closely resembles it when seen from a short 

 distance that it has elsewhere been described as litnestofie con- 

 taining crystals of hornblend. It constitutes the leptinite of 

 Brogniart. 



Mica in nearly all its varieties occurs in many places, and 

 it is much to be desired that the different kinds should be in- 

 vestigated, the more especially since the very interesting optical 

 properties of this subgenus have been discovered. For our pre- 

 sent purpose, it is only necessary to notice a few of the most 

 interesting localities. On the Patapsco (the ^prismatique and 

 bitiaire'') crystals of Haliy occur in large grained granite. On 

 Jones' falls, 2h miles from the city, in a similar granite, pris- 

 matic crystals abound, but their sides are much obliterated. 

 One remarkable property of these crystals is that when viewed 

 through the sides of the prism, they are translucent, even if 

 more than one inch broad; yet they are perfectly opaque 

 across the laminas, when less than the twentieth of an inch in 

 thickness. In the same vicinity, the mica in a coarse granite 

 exists in masses of a cuneiform shape of considerable size and 

 with a peculiar structure, which may be understood by sup- 

 posing the annexed diagram to represent a section through 



the mass. The lines represent 

 the laminas, although they are 

 not so uniform, but are partially 

 interlaced. 

 A-^^^^^^^^^^^^J Upon separating a portion of 



the laminse and holding them up 



to the light, lines of a dark colour 



appear, forming part of two or 



more sides of a regular hexahe- 



dral figure, whose centre would 



be at the apex of the wedge, marked (A). It is probable that 



the form of the pieces is the result of the operation of a regular 



law of crystallization, which has not been developed. 



In another spot in the same vicinity, mica of similar form 

 embraces jorecioMs garnets, whose crystallization has been in- 

 terrupted by the plates of mica, so as to have produced flat 

 crystals ; some of which are nearly a quarter of an inch broad, 

 the thickness is variable, some not being thicker than writing 

 paper. They have not been flattened by pressure while soft, 



