352 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



of these schists and argillites, and if it had extended across this depression 

 ti would have covered the beds from which it must have derived its 

 material. It would then have had the gneiss for its shore country, and the 

 gneiss must have supplied a large part of its mass. On the contrary, these 

 Pelliam biotite-gneisses seem wholly wanting in the Mount Toby conglom- 

 erates, from which I conclude not only that the schists and argillites then 

 filled this depression and furnished the conglomerates a border nearly on 

 the present boundary of the latter, but that they then mantled eastward 

 over the gneiss. This demands an unexpected amount of erosion during 

 and since the Trias. 



The artesian wells that have been bored along the line of the Connecti- 

 cut with depths from 600 to 3,700 feet have never reached the bottom of 

 the sandstone. (See p. 380.) If we add to this the height of the crystal- 

 line walls of the valley above the Connecticut, I think we may estimate the 

 present depth of the Triassic trough at somewhat above a mile. Indeed, I 

 shall show that the major portion of the material of the Triassic beds came 

 from the immediate borders of the basin, and Avould thus add another con- 

 siderable but unknown quantity to the maximum depth of this long and 

 naiTOw trough. I thinlc the maxinuim thickness of the Triassic beds therein, 

 restoring the post- Triassic and especially the Grlacial erosion, must have 

 been considerably more than a mile. 



I have elsewhere (see p. 13) discussed the system of faults bounding 

 the block, or group of blocks, whose sinking formed this Yosemite-like 

 Triassic valley, or "graben," to use the nomenclature of Eduard Suess,^ and 

 their outer boundary can be closely followed on the new four-sheet map of 

 Massachusetts by tracing the 500-foot contour line at the foot of the escarp- 

 ment east and west of the Connecticut, though the fault lines lie generally 

 a little lower — that is, nearer the river. 



If this line be followed from the north line of the State just east of the 

 Connecticut to the Belchertown ponds, and another line be drawn down 

 the Connecticut to the mouth of Millers River and south to Mount Tom, it 

 will include a long quadrangular area having its base at the northern foot 

 of the Holyoke Range, which area was once deeply covered by the Trias, 

 but has now been for the most part denuded of this covering. Over this 

 area the crystalline substratum of the valley stands everywhere about 300 



' E. Suess, Das Antlitz der Erde, p. 166. 



