269 



Mineralogical JSfotes. 



Pinite. — The abundant occurrence of pinite in the pebbles 

 and paste of some parts of our Primordial conglomerate, and the 

 probable origin of this pinite debris in the Huronian petrosilex 

 and felsite of this region, have been noticed on pages 89 and 220. 

 It is proposed here to give (1.) a more detailed statement of 

 the relations of this mineral (or rock, as it is perhaps more 

 properly called) to the petrosilex, especially in Milton; (2.) 

 chemical analyses of specimens taken from both the petrosilex 

 and conglomerate, showing that this material is a true pinite, 

 and that its composition is essentially similar in the two geo- 

 logical positions; (3.) further observations on its distribution 

 in the conglomerate, together with a notice of its occurrence in 

 the other fragmental rocks of Eastern Massachusetts. 



(1.) Wherever occurring in the conglomerate, the pinite, as 

 already observed, is always clearly an imported constituent ; 

 but in its association with the petrosilex and felsite it never 

 presents this aspect ; all the facts pointing to the conclusion 

 that it is indigenous in this formation. In other words, the 

 pinite, so far as the evidence allows us to judge, exists in the 

 petrosilex only as a product of the superficial decomposition or 

 alteration of that rock. Indications of this may be observed 

 in many places. For instance, I have remarked on page 79 

 that the exceedingly homogeneous greenish and grayish elvan- 

 ite, so extensively developed in Needham, very commonly pre- 

 sents a slaty appearance and yields to the knife ; and this is 

 probably a superficial phenomenon. Substantially the same 

 language is repeated on the next page in describing the green 

 petrosilex in West Dedham ; also, on page 88, of the petro- 

 silex exposed on River Street in Hyde Park, and on Blue Hill 

 Avenue in Milton. And the greenish " toadstone " and 

 other varieties of petrosilex in Newbury may be placed in the 

 same category. In all these cases the rock is green, at least 

 superficially. In Marblehead, Lynn, and other districts I have 

 observed the brown, gray, black, and other colors of the petrosi- 



