MUSEUM OF COMPARATIVE ZOOLOGY. 117 



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 diifer materially in their uvcroscopic characters from those 

 of West Eock. 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 coiatains 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 augite 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 ibid., p. 190. 



3 Ibid., VI., 1873, p. 107 



