TONALITE. 341 



the chalcedonic quartz, colored a light pistachio-green by epidote. These 

 surfaces are not "slickensides," but are as if varnished, and are caused by 

 the crystallization of the fine-grained material. They are analogous to the 

 smooth surface of botiyoidal chalcedony or liraonite. 



A similar peti'osiliceous variety occurs in Whately — a j)ale leek-green, 

 subgranular mass, of homstone-like appearance, with a few crushed mus- 

 covite plates. It shows no biotite or quartz. The luster is generally dull, 

 but here and there the sheen of a feldspar cleavage ajjpears, and this 

 always shows tri clinic striation. It appears at the Hatfield lead mine 

 in thin layers on fissures. (XVIII, No. .')?, in Massachusetts Survey 

 Collection.) 



PETKOGEAPHICAL DESCRIPTION OF THE ALTERED TONALITES. 



'' Shay's flint," Pelham, the typical rock. Under the microscope this 

 shows a regularly mottled aggregate polarization which has some resem- 

 blance to clastic structure, but more to that of agate or chemically deposited 

 quartz; and as it resembles exactly the purer parts and the veins of the 

 same hornstone from Pelham, in situ, I have no doubt that it is chemically 

 deposited silica, rendered impure by kaolin and a little green chlorite. It 

 is in large part apolar, and therefore opal. 



Tonalite, Pelham, west line north of S. Jewett's. Dark hornblende 

 abundant, feldspar flesh-colored. In section very feldspathic, the feldspars 

 (mostly tri clinic) greatly kaolinized; all constituents reach the extreme of 

 crushing — the hornblendes opened along cleavage planes; the feldspars 

 crushed and parts moved; the twin striation greatly twisted, and the 

 undulatory extinction greatly obscuring the twinning; hornblende shows 

 jc = gi-een, 13 = olive, a = yellow; c=b>a. 



Green hornstone, like "Shay's flint," from locality where first found 

 by me in place in Pelham, at S. Jewett's. This is a quartz mass, filled with 

 scales like kaolin, which are opaque by transmitted and white by reflected 

 light, and permeated by veins which have the same scattered scales. It 

 resembles exactly, both with and without polarized lig'ht, the time "Shay's 

 flint." Some slides show a beautiful microbrecciation from crushing. They 

 contain magnetite and a little green chlorite. Under the polarizer there 

 appear now and then larger, rounded, transparent grains, which may be 



