516 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



of the Trias were much higher than now or that the sandstones once 

 extended much farther east and west. 



Mount Toby, wholly made up of sandstone and conglomerate, is 1,275 

 feet above sea level (1,170 feet above the river), but to obtain its true 

 height as a measure of the height of the Triassic at its maximum we must 

 add a considerable but unknown amount for subaerial and glacial erosion. 

 It is possible that we must subtract something also to offset the elevation of 

 the mass during the disturbances which have tilted the rocks. I imagine 

 it would be an overestimate of the latter movement if we should assume it 

 to have been great enough to counterbalance the depression of the old sur- 

 face by erosion. If, then, we take the present height of Mount Toby or 

 Mount Tom as that of the sandstone at the close of its deposition and run 

 a contour line at this level along the sides of tlie valley to obtain the orig- 

 inal boundaries of the sandstones, on the assum{)tion that the valley walls 

 were then about their present height, this line would lie so far back from 

 the present border of the sandstone and run into so many sheltered valleys 

 that we should encounter gi-eater difficulty in explaining why the sandstones 

 are wholly absent from these broad areas on each side of their present 

 limits than in assuming a very considerable degradation of the walls of the 

 valley since it served as an estuary for the accumulation of the Triassic 

 sediments. Indeed, we may say directly that the present border of the 

 sandstones represents closely the old border of the estuary, because the 

 coarse angular conglomerates and rudely sorted feldspathic sandstones can 

 have been transported but a very short distance, and, as their mineralogical 

 cliaracter indicates, must have been derived largely from beds immediately 

 adjacent, which would have been covered if the waters of the estuary had 

 extended as far as the supposed contoui* line, and that, therefore, the crys- 

 talline border of the valley must have been sufficiently higher than now to 

 form retaining walls for the accumulated Triassic gravels. Within these 

 limits the coarse sandstone rose, as above indicated, to a height above that 

 of Mount Toby, filling the whole valley to that level. The increased ele- 

 vation may have amounted to many hundred feet. 



It would be interesting to follow the com'se of the erosion by which the 

 present ridges have been sculptured out of this mass and to divide the long 

 quiet work of the waters in later Mesozoic and Tertiary times from the work 

 of the harsher agencies of the Glacial period. 



