B. K. Emerson — Calcite- Pre finite Cement Rock. 277 



Art. XXY. — Note on a Calcite- Prehnite Cement Rock in the 

 Tuff of the Ilolyoke Range ; by B. K. Emerson. 



At Lymans Crossing, now abandoned, a mile north of 

 Smiths Ferry, and just south of the river notch through the 

 Holyoke Range, is a large cutting through the posterior trap 

 sheet, exposing the upper surface and the superjacent tuff 

 beds. The cementing material which holds the tuff fragments 

 together at the base of the bed is quite peculiar. It looks like 

 a felsite or a compact sedimentary limestone. It is clear gray 

 with faint shade of green. 



The small angular fragments of trap enclosed in this cement 

 are often one to three inches apart, showing that it cannot be a 

 simple secondary interstitial cement produced by a later infil- 

 tration. It contains here and there rounded or pear-shaped 

 cavities, filled with coarse calcite, which seem to be certainly 

 steam holes. Minute scales of graphite just visible to the eye 

 are quite generally distributed and are slightly larger and more 

 abundant where the cement rock borders against the trap. The 

 scales are graphite and not molybdenite, since they float in 

 Thoulet's solution. 



Under the microscope the trap fragments are seen to be 

 normal and to preserve their usual characters up to their 

 borders. 



The cement rock has a confused crystalline texture and 

 large stationary black crosses appear everywhere over the sur- 

 face. It is made up in about equal parts of calcite in shapeless 

 areas with very irregular boundaries, and a colorless prehnite 

 in coarse rudely radiating prisms and wheel-shaped forms 

 which plainly cause the black crosses. A few blades of a pale 

 brown biotite are present but may be secondary. Distinctly 

 secondary are the angular fragments of acid plagioclase and 

 microcline, which have a granitic aspect. The rock has sp. 

 gr. = 2*86, which indicates that a little more than half its mass 

 is prehnite, and the study of the section confirms this. The 

 small crumpled graphite scales are also secondary, and must 

 have come from west of the axis of the Green Mountains, 

 twenty-five miles west, or from the Brinefield rusty schist 

 area fifteen miles east. The brightly shining scales resemble 

 those from the western area. The same graphite is found 

 extensively in the adjoining sandstone. There is also a small 

 amount of a primary albite deposited by the same waters from 

 which the prehnite crystallized and having the same undulose 

 extinction which characterizes the albites deposited by heated 

 waters in the cavities in the trap. 



