424 PECK 



The first of these faults extends along the northwestern side of 

 Marble Mountain and Chestnut Hill and constitutes the bound- 

 ary line between the pre-cambrian and the post-Algonkian (Plate 

 XVI. ^-C). It is a typical break thrust fault. At A there is a 

 branch fault given off which runs along the southeastern slope 

 of Chestnut Hill to a point B where it appears to die out. It is 

 along this branch fault AB which cuts, as it w^ere, diagonally 

 across the ridge that all the talc and serpentine deposits of the 

 region, with one exception, occur.^ It partakes here more of 

 the nature of a stretched thrust and all of the rocks involved in 

 the faulting are so sheared, stretched and profoundly altered as 

 often to be recognized with difficulty. The granites, for exam- 

 ple, have every particle of their feldspar constituent sheared into 

 a stringy, almost fibrous variety of sericite, not a trace of the 

 original feldspar remaining, while the quartz remains as lenticular 

 or film-like patches but crushed almost to a powder. The horn- 

 blende alters for the most part to serpentine, in fact the richly 

 hornblendic granites seem to furnish one source of serpentine. 

 The calcite -dolomite beds shear to a slaty, foliated, talcose mass 

 consisting of a mixture of talc, tremolite, serpentine, with occa- 

 sional seams of tremolite (?) asbestos, imbedded in a fibrous car- 

 bonate of lime (aragonite ?), the fibers of both minerals lying 

 parallel to each other and normal to the walls of the seam ; or 

 without shearing, become changed to beds of nearly pure white 

 tremolite. The phlogopite which is developed locally in large 

 quantities in connection with these beds, sometimes in huge 

 crystals a foot in diameter but usually in rather finely granular 

 masses, alters quite uniformly to serpentine and constitutes the 

 chief source of that mineral in the Easton quarries. Huge 

 masses of it weighing many tons are removed from time to time, 

 consisting quite wholly of phlogopite mica in different stages of 

 alteration to serpentine. Locally these masses of phlogopite 

 become changed to the very finest variety of ** Royal " of " Vic- 

 toria Serpentine " which occurs here in such abundance as to 



* The principal occurrences of talc and serpentine are indicated by a widening of 

 the fault lines along which they occur and quarries from which they have been taken 

 are numbered I -7, 



