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LITHOLOGICAt OBSERVATIONS 



1. The penetration of the rock by veins and 

 masses of pure quartz, has been remarked. It is 

 well seen in the clay-slate quarries ; and it leads 

 to the inquiry, whether the quartz, in the form of 

 nests and small masses, belongs to the original 

 structure of the stone, or is a foreign ingredient 

 subsequently injected in a fluid state, by an ex- 

 ternal force acting from below. Now, although 

 the whole phenomena may be difficult of ex planar- 

 tion on either of these two principles, the one first 

 mentioned appears to me the most natural ; be- 

 cause the irregularity in the structure of the stone, 

 which accompanies the occurrence of the quartz, 

 is also to be observed not less frequently, where 

 no separate portions of quartz are to be seen at 

 all. It is a circumstance not a little favourable to 

 this idea, that masses of quartz perfectly isolated 

 may be every where observed, with the layers of 

 the rock in which they are immersed, lapped 

 round them, and obeying exactly the irregularis 

 ties of their various shapes, 



2. The internal structure of the pure mica-slate 

 itself, furnishes another remarkable subject of 

 theoretical discussion. Betwixt the layers, which 

 observe the general direction of the strata, we 

 find others in which the concretions are at diffe- 

 rent angles to the direction of the including lay- 

 ers, or follow waving irregular lines, in an end- 

 less variety of forms. At times, these layers 



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