220 



the river, the conglomerate has quaquaversal dips ; and at the 

 western end it is curiously involved with the semi-exotic amyg- 

 daloid, of the debris of which it is principally composed. At 

 the eastern end, on Central Avenue (a new street not shown 

 on the map), some of the conglomerate is a coarse irregular 

 breccia, with layers of finer material which show a high dip to 

 the south-east. The most interesting rock at this point, how- 

 ever, is a conglomerate composed mainly of a brownish, slaty 

 paste, enclosing many small pebbles of a soft, greenish, and 

 somewhat unctuous substance, which is some form of the 

 mineral pinite, — a hydrous, alkaline silicate of aluminum, — 

 appearing to come nearest to the variety agalmatolite. 



This mineral, which most observers have mistaken for serpen- 

 tine, is of comparatively frequent occurrence in the conglomer- 

 ate of the Boston basin, though nowhere so abundant as in 

 this Milton locality. At some points the paste or cement of 

 the conglomerate appears to include much pinite, yet in its 

 purest state this substance occurs mainly in the form of peb- 

 bles ; and these, on account of the inferior hardness of the 

 material, are usually very much flattened in parallel planes, as 

 if by pressure ; giving rise, where they are sufficiently abun- 

 dant, to a decidedly schistose structure in the rock, or, more 

 properly, an imperfect cleavage. 



Pinite conglomerate, having a strongly marked cleavage, is 

 well exposed in a cut on Central Avenue, about a quarter of a 

 mile south of the railroad, where it lies upon, and may be seen 

 in contact with, the purplish petrosilex described on page 89 ; 

 which appears to have been, by a species of sub-aerial decompo- 

 sition, partially converted into a soft, greenish, hydrous alumi- 

 nous silicate, precisely similar to that so abundant in the con- 

 glomerate. In fact, it is easy, as remarked on the page just 

 cited, to see whence the pinite of the conglomerate, which is 

 very clearly an imported constituent, has been derived. For 

 farther observations on this point the reader is referred to the 

 mineralogical notes that complete this account of the Primordial 

 formation. 



