Structure of Mica Slate, fyc. 



337 



the strata correspond with those of that range : and indeed I have 

 traced it nearly to the place of connection with the Worcester mica 

 slate. Bat commonly it is more decided in its characters, and more 

 crystalline in its texture, than the rock of that range generally ; espe- 

 cially where it approaches the coarse granite of Andover. 



Slaty and Stratified Structure of the Mica Slate, with the Dip and 

 Direction of the Strata. 



It is rare to find even a small portion of this rock destitute of a 

 schistose structure. But it is not uncommon to meet with extensive 

 masses in which it is very difficult to trace any strata seams. In 

 other places, however, no rock exhibits more regularity and beauty 

 of stratification. Such differences may in general be explained by 

 local disturbances ; but sometimes no appearances will warrant such 

 an explanation of the phenomena. And it seems reasonable to im- 

 pute something to different proportions of the ingredients in the rock, 

 and to peculiarities in the mode of formation. In general, the less 

 the quantity of mica, the more regular is the stratification. The 

 mica slate in Goshen, Chesterfield, &c. which is remarkably regular 

 in this respect, consists, however, chiefly of mica. Those varieties 

 exhibit most of contortion and undulation in the layers, which are of a 

 plumbaginous aspect, and contain tuberculous masses of quartz. 



Though our mica slate is sometimes divided by a double set of 

 strata seams, yet I have never met with any examples in which the 

 planes of stratification make an angle with the laminae of the slate. 

 The flexures of the laminae, however, seem in a great measure to be 

 independent of the planes of stratification ; and the two structures ap- 

 pear to have resulted from different causes. The two following 

 sketches will illustrate this. In each case a single stratum only is 

 represented ; and it will be seen that the curvatures of the laminae 

 have no connection with the strata seams, which are represented by 

 the parallel lines that include the contorted layers. In these cases 

 the strata are two or three feet thick : and they are generally thick ; 

 sometimes eight or ten feet, where the slate is much bent. The stra- 

 ta are often bent as well as the laminae, and sometimes the curvatures 

 of the former are parallel to those of the latter. 



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