G. W. Hawes—Trap Rocks of the Connecticut Valley. 191 
South DurHaM Mountain. Sp. gr. = 2°83. 
Mean 
Siliea.scuxdaieien 46°56 46°51 46°54 
Alumina <J..+<% 14°75 15-05 14°90 
Ferrous oxide... 9°89 9°78 9°83 
Ferric oxide - 3°58 3°50 3°54 
sacar oxide °33 “35 “34 
Secs ee ce 79 7°90 7°94 
Tisnele Mpg SE 4°83 4°89 4°86 
2°47 2°38 2°43 
Potash <)> 2...:; 59 60 60 
Water oo 4°50 4°54 4°52 
Carbonic acid... 4°38 4°32 4°35 
99°87 99°82 99°85 
the time of ejection, as has been urged by Prof. Dana.* If the 
trap-rocks of this eastern region, when comin elt 
through the red sandstone strata, encountered jataieranees 
waters, and if it would be impossible for the vapors produced 
by the heat to be pressed back, owing to the hydrostatic pres- 
sure above, then these vapors, ‘together with other vaporizable 
material, as carbonic acid, set free from its combinations in the 
strata by the heat, would pass into the mass of molten matter. 
In this way we have a sufficient explanation of the change that 
made this diabase out of the material that formed the dolerite. 
e pyroxene was the mineral that was most attacked, the 
result of the alteration being chlorite. As chlorite is a magne- 
sian mineral, lime was set free by this change. Part or all of 
are common in the cavities. The exact sremie reactions that 
took place will be considered at another time, when more of 
the products of decomposition have been sini: That there 
was such a passage of he ug saat the molten mass is evi- 
dent; for the r ork at contains long pipe-stem-like 
cavities, which were i byt he ascending vapors, and which 
are generally filled with calcite 
* See this Jour., IIT, vi, 104. “J. D. Dana on Igneous Ejections and Volcanoes.” 
Am. Jour. Scr.—Tairp oe Vou. IX, No. 51.—Manrcu, 187, 
