248 GEOLOGY OF OLD HAMPSHIEE COUNTY, MASS. 



All the constituents are perfectly fresh and almost entu-ely free from 

 fluid inclusions and microlites, and the absence of these, as also of zircon, 

 rutile, garnets, and iron ore, is remarkable. 



Hand specimens are on one side biotite-gneiss, on the other ejjidosite, 

 and the two seem normally interlaminated ; but the latter must be of later 

 and very different origin, and may be in effect a vein stone, in which, per- 

 haps, the chlorite scales are remnants of the earlier rock, which has been 

 almost wholly resorbed to make place for the new minerals. 



THE WIIiBRAHAM SYKCLINE. 



South of the deep transverse valley of the Quabaug and its continua- 

 tion in the Chicopee River, the simplicity of the geology is as marked as is 

 the complexity of the region north of the same valley. Three great syn- 

 clines of the schists run south across the towns named above, forming as 

 many high ridges. The Wilbraham syncline looks down on the sands of 

 the Connecticut Valley on the west and upon the deep gneiss-bottomed 

 valley of East Wilbraham on the east, and across this valley rises the West 

 Mountain of Monsou, made up of a second syncline of the same rocks and 

 looking down on the deeper and narrower Monson Valley, which is under- 

 lain by the same gneiss. Across this valley on the east the third syncline 

 rises to form East Moi;ntain, which is bordered on the east by a less strongly 

 marked and yet distinct valley, underlain by a third repetition of the 

 Monson gneiss and, followed farther east, by the Brimfield gneissoid mica- 

 schists, forming a fourth syncline. (See sections, PI. XXXII, and map, 

 PI. XXXIV.) 



The Wilbraham syncline is concealed in its western half beneath the 

 Triassic sandstones, which rest against the western foot of the ridge, and 

 the slope of the ridge on the west is so steep that it is probable that the 

 fault, so well marked farther north, is continued at its base, and that the 

 rocks have sunk to form the broad Connecticut Valley. It is a closed 

 fold, slightl}' overturned to the west, and its rocks closely resemble the 

 corresponding beds on the west of the Connecticut Valley in Granville. 



The gneissoid quartzite or muscovitic gneiss, the equivalent of the Rowe 

 schist, which usually intervenes between the Monson gneiss and the horu- 

 blendic beds, seems to be wanting here, and the hornblendic beds rest 

 directly on the white biotite-gneiss. The upper beds of this gneiss are very 

 fine-grained and magnetitic and probably represent the Rowe schist, but 



