MUSEUM, OF COMPARATIVE ZOOLOGY. tz 
graphically by Hawes, and classed by him as a dolerite.’ Sections from 
near the middle part of the trap sheet forming Gaylord’s Mountain do 
not appear to differ materially in their microscopic characters from those 
of West Rock. The trap is holocrystalline far from its upper and lower 
junction with the sandstone or shales, and, as has been pointed out by 
Hawes, is much less altered, and contains fewer hydrated minerals, the 
products of decomposition of the augite, feldspar, etc., than the erup- 
tive masses forming Saltonstall Mountain, or the Durham range, to the 
east. Hawes believed this difference to be connected with geographical 
location, and thought it had nothing to do with geological age.” Ac- 
cording to J. D. Dana,’ the great alteration of the trap in the eastern 
range took place at the time of ejection, and depended on the en- 
countering of subterranean waters which the molten rock took up in 
its passage through the sandstone strata. Hawes followed this view, 
and thought the eruptive magma might in such a way assume the 
diabase type, while under less humid conditions the same magma on 
consolidating would form a dolerite. 
It appears, however, that the difference in the hydration of the east- 
ern and western traps can be better accounted for by original structural 
and mineralogical differences incident to the very different conditions 
under which the several trap sheets solidified. This will be referred to 
again in the special account of Saltonstall Mountain. 
In the trap from Gaylord’s Mountain, on approaching the overlying 
sandstone, there is a gradual fining of the texture and an increased ten- 
dency towards a porphyritic structure, the porphyritic crystals there 
being set in an undifferentiated, non-polarizing base. The augite occurs 
more rarely in well-outlined individuals, and constantly tends towards a 
granular structure. Olivine, which has been detected in minute grains 
in the same rock to the south, has once been abundant at the Roaring 
Brook contact, in well-outlined porphyritic crystals, but is now mostly 
altered to a fibrous grass-green to yellowish-green serpentine, or entirely 
replaced by pseudomorphous calcite or dolomite. The augite occurs 
less and less plentifully upwards, and at two inches from the junction 
with the sandstone it cannot be found even in grains. Accompanying 
the loss of angite and the increase of olivine, there is, especially at the 
contact, a development of a non-polarizing base in which are scattered 
innumerable acicular ledges of feldspar, some porphyritic, showing an 
1 Amer. Journ. Science, IX., 1875, p. 186. 2 Thid., p. 190. 
3 Ibid., VI., 1873, p. 107. 
