48 



GEOLOGY OF OLD HAMPSHIEE COUNTr, MASS. 



gneiss; and during tlie strong inetamorphisni whicli has transformed the 

 conglomerate into a gneiss, the saxonite has been largely changed into 

 anthophyllite, and, what is of higher interest, a broad selvage — a "reaction 

 rim " on a grand scale — has formed by the mutual influence of the basic 

 core and the acid surrounding. This selvage consists largely of minerals 

 containing little or no silica — apatite, corundum, magnetite, tourmaline, 

 anorthite, and biotite. It wraps around the saxonite with great irregularity, 

 often folding deeply into its mass. 



]\Ieasured downward from the hanging wall, the mass is greatly decom- 

 230sed for a depth of from 3 to 12 feet, and as the pits have been sunk in 

 this decomposed ]>ortion and work stopped as the hard unaltered rock was 





Fig. 3 — Southwest wall of Pelham asbestos quarry in 18aO. A, Pelham gueiss; B, anorthite rock; C. black 

 tourmaline masses ; D, biotite layer ; D', vermiculite layer (vermiculite and steatite from biotite and actinolite) ; E, saxonite ; 

 F, anthophyllite derived from saxonite. 



approached, attention has been directed almost entirely to this decomposed 

 portion, which will be discussed a little later. The pits everywhere 

 disclose at the bottom the black rock, which consists of granular olivine, 

 chromite, magnetite, and brouzite (saxonite), without admixture of any 

 other minerals, and of this rock the whole intruded mass originally 

 consisted. 



At the top of the wall in the large central cutting the contact of the 

 superincumbent gneiss upon the olivine rock can be well studied, and it is 

 very peculiar. (Fig. 3.) The gneiss penetrates the olivine rock in a great 

 club-shaped apophysis, the lamination of the gneiss being first bent down 



