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The greenish sandstones and slates of this formation are also, 

 probably, often rich in pinite, and the same may be said of 

 the slaty beds in the Shawraut group. While in many places 

 the petrosilex-breccia of the Shawmut group embraces, in both 

 pebbles and paste, a large proportion of what appears to be 

 more or less perfect pinite; i.e., material of a light-green, or 

 greenish-white color, which yields to the knife, and is some- 

 what unctuous, resembling serpentine in its external characters, 

 and yet readily shown by its easy fusibility to be aluminous and 

 not magnesian. Dr. Hunt has called my attention to the exist- 

 ence of pinite in the breccia in Saugus, and my own observa- 

 tions have convinced me that its occurrence in this way is a 

 general fact. 



To furnish all the pinite debris of these various fragmental 

 rocks an extensive formation has been required. No vestiges 

 of such a formation, distinct from the petrosilex, now exist in 

 this region ; and with the petrosilex we have the pinite, so far as 

 the evidence allows us to judge, only as a product of the super- 

 ficial decomposition of the former rock. The facts seem to 

 warrant, or at least to forcibly suggest, the conclusion that, in 

 pre-primordial times, the petrosilicious rocks, to a considerable 

 depth, were changed by the action of atmospheric agents, not to 

 kaolin, as generally at the present time, but to or toward pinite ; 

 and that subsequently this decomposition-product was for the 

 most part swept away by the sea, in which were deposited the 

 Shawmut breccia and Primordial conglomerate. 



Kaolinite. — At the large quarry on Tremont Street, a short 

 distance west of the Roxbury Station on the Boston and Provi- 

 dence Railroad, the conglomerate is traversed by several narrow 

 veins and irregular cavities filled with a soft, white, kaolin-like 

 substance, to which my attention was first called by Prof. 

 W. R. Nichols ; and I am indebted to him for the following 

 notes and analyses made nearly nine years ago : — 



*« The mineral varies from a soft, powder-like kaolinite to a 

 quite hard massive variety. From the appearance, and from 

 the reaction before the blow-pipe, I judged it to be kaolinite ; 



