196 B. K. Emerson — The Deerfield Dyke and its Minerals. 



but fold around all small protuberances in its upper surface as 

 if it were a later deposit. This is shown in an almost un- 

 broken outcrop a mile long opposite Turner's Falls, where the 

 river has notched into the trap and exposed the junction, 

 facts can hardly be explained except by supposing the 

 have been poured out upon the lower sandstone, and 

 3 been covered by the upper shales. The same state of 

 things holds with regard to the great Holyoke dyke, and in 

 the extensive tufa beds intercalated in the sa 



:";,:" 



the latter and described by President Hitchcock 4 

 it strongly reinfoi m. I am aware tl 



% the 



from the blocks of diabase enclosed in this 

 stratum at the most accessible locality of it, the roadside below 

 Smith's Ferry in Northampton and find it to be identical with 

 that of the Holyoke range immediately north. The blocks 

 are large, angular and abundant, and are mixed with granitic 

 material and I cannot doubt that they have come from the 

 erosion of the great dyke, which must thus antedate them. 



Similar tufa beds are exposed above the amygdaloid of the 

 dyke under discussion at the west and highest point of the 

 outcrops on the Greenfield road. They are here of finer grain 

 and closely resemble the Nassau Schalsteine. 



Lithology. — The rock is a typical diabase, ranging from 

 aphanitic varieties to those where the flat white feldspars are 

 2 V-' ■ square, and from compact to very coarse amygdaloidah 

 The different varieties are of very uniform texture and always 

 in an advanced stage of decomposition though appearing quite 

 fresh ; plagioclase apparently of two species ; augite, magne- 

 tite and olivine are uniformly present. Apatite cannot be 

 detected. 



The common plagioclase, probably labradorite, is always by 

 far the most abundant constituent, and the angle of extinction 

 of its long rod-like crystals is commonly 12°. Several vari- 

 eties of the rock are sub-porphyritic by the development of 

 white spots made up of groups of stout crystals of a second 

 triclinie feldspar apparently distinct from the first whose angle 

 of extinction is 21°. Both feldspars are thoroughly decom- 

 posed commonly from the center, and sometimes show only 

 aggregate polarization. 



The augitic constituent has for the most part gone over into 



a mixture of green and brown chloritic minerals, here and 



there an exceptionally large crystal remains in whole or part 



* Geology of Mass., 1841, p. 442. 



