Varieties of Clay Slate. 



287 



the map, there can be little doubt but they belong to De la Beche's 

 Inferior Stratified or Non-fossiliferous Rocks ; though I doubt not 

 but some of them would be placed by a Wernerian geologist among 

 the " transition clay slates." But I regard it as quite unprofitable to 

 enter upon the long agitated question whether our clay slate belong 

 to the transition or primary class of rocks. It will be in season to dis- 

 cuss this point, when any one can show where the transition class be- 

 o-ins or ends; and when there shall be even a tolerable agreement as to 

 the place in the series where the primary class commences. A mere 

 inspection of the tabular view of our rocks, which I have given in the 

 Atlas, will show that the answers to these questions will depend upon 

 the system or classification which we adopt. I shall feel satisfied, if 

 I can correctly describe the position of this rock in respect to others. 

 In what follows, I shall leave out of the account the argillaceous slate 

 in the vicinity of Boston. 



Miner alogical Characters. 



The common argillaceous slate, which in its most perfect state 

 forms roof slate, is the only variety of importance belonging to this 

 formation. This passes by slow gradations into mica slate; so that 

 it is often impossible to say where the one terminates and the other 

 commences. Hence some of the specimens which I place under 

 mica slate, other observers would place under argillaceous slate, and 

 vice versa. 



The laminae in the best varieties are straight and even : but as it 

 approximates to mica slate, they become minutely undulated, the sur- 

 face resembling exceedingly that of certain shales of the new red 

 sandstone already described. Some of these intermediate varieties 

 are remarkably contorted: but these I shall describe under mica 

 slate. 



In Guilford, Vt. through which the Franklin County range of this 

 slate extends, I have observed that it passes into a fine grained vari- 

 ety of chlorite slate, and even perhaps into novaculite. That range 

 also abounds with tuberculous masses of white quartz. Veins of 

 quartz also occur in it, as I shall have occasion shortly to show more 

 particularly ; and in Guilford we find protruding masses of a por- 

 phyroid granite, passing into compact feldspar, and a slaty mixture 

 of this last mineral and quartz. The Worcester County range, also, 

 contains granite. 



