Amphibolic Aggregate. 



259 



I have no doubt that this argillaceous slate is the ' transition clay 

 slate' of the Wernerians, which they describe as associated with gray- 

 wacke. In various places in Rhode Island and Massachusetts, it is 

 highly impregnated with carbonaceous matter, so as to become black ; 

 and it usually forms the floor and roof of the beds of anthracite. In 

 general its color is dark gray, passing to blue : sometimes of a cho- 

 colate color, and sometimes red. It is rarely fissile enough to be 

 employed for roofing, and frequently its layers are two or three inches 

 thick. The laminae, or rather strata, are sometimes much curved ; 

 as on Rainsford Island, in Boston Harbor. Not unfrequently it passes 

 into an imperfect novaculite ; as in Charlestown, Roxbury, Wey- 

 mouth, Newbury, and some of the outer islands in Boston Harbor. 

 (Nos. 357 to 370.) 



8. Amphibolic Aggregate. (No. 374.) Nothing is more difficult 

 in many cases, than to determine the nature of the semi-crystalline 

 minerals entering into the compositon of some of the intermediary 

 rocks. They seem to have undergone some chemical process, 

 which has not been thorough enough to give them a fully devel- 

 oped character. In the present instance the mass appears decidedly 

 crystalline; yet I am in serious doubt whether amphibole is the 

 dark green mineral in it that exhibits a crystalline structure. An- 

 other part of the rock presents an argillaceous aspect, and exhales 

 an argillaceous odor when breathed upon. But had I found it among 

 primary rocks, I should have regarded it as by no means an anomaly 

 there : especially after finding in it a vein, four inches wide, of crys- 

 tallized zoisite. Yet the position of this rock, which has already 

 been pointed out, in describing the conglomerate of the southeast part 

 of Rhode Island, clearly proves it to be a member of what I call the 

 graywacke series : for it is situated between graywacke slate and 

 conglomerate. 



9. Varioloid Wacke. The rock which I thus designate, has gener- 

 ally been regarded by those who have described the Geology of Boston 

 and its vicinity, as amygdaloid. But it seems to me that there are in- 

 superable objections against the supposition that the nodules in gen- 

 eral were introduced by infiltration, or even sublimation ; the only 

 modes by which geologists suppose the cavities of amygdaloid were 

 filled. For they consist generally of rounded masses of compact 

 feldspar ; a substance which must certainly have been the result of 

 igneous fusion. On the other hand, the rounded form of these nod- 

 ules, and their non-crystalline structure f in general, forbid the arrange- 

 ment of this rock along with the porphyries. But some writers re- 



