COXTEMPORAXEITY SHOWN BY TRAP SHEETS. 455 



which swept up the western and down the eastern side and left the center 

 of broad shallow mud flats at a considerably higher level than the shoreward 

 portion, so that they alone were regularly abandoned by the water at low 

 tide. It follows from all this that the deposits named above were contem- 

 poraneous, and this is neatly shown by the trap sheets. The Holyoke sheet 

 came up through a fissure in the shale and flowed westward out over the 

 brownstoue and the arkose ; but for miles, while resting on these latter rocks, 

 its basal portion is full of fragments of shale and limestone, picked up while 

 flowing over the mud beds. The Deerfield bed rests at its northern and south- 

 ern ends on the Mount Toby conglomerate, in its middle upon the sandstones 

 and shales, and beneath the lookout tower east of Greenfield it plainly 

 flowed with a thickened crust, which unrolled before it and became deeply 

 fissured as it came to lie beneath the thick mass, so that by hydraulic press- 

 ure the red mud was forced five or six feet up into the fissures of the porous 

 trap from below ; while, immediately above, the same fissures are filled for 

 another six feet by an aphanitic trap, which oozed down from the still 

 melted mass above, cementing the porous blocks. The great thickening of 

 the Holyoke bed in its western portion. Mount Toby, may be due to its 

 flowing westward into the deeper channel. 



The Monoclinal Faulting. 



Having made out in general the distribution of the different varieties dis- 

 tinguished above, the boundaries were studied with more care, with fuller 

 consideration of dip and strike and with special reference to the question of 

 monoclinal faulting. The only part of the area suitable by the character of 

 the outcrops and the direction of the boundaries was the northern portion 

 of the Turner's Falls sandstone area, where the boundary runs east and 

 Avest ; and northward of and parallel to it is the outcrop of a trap sheet to 

 repeat any faults which might be found in it. This boundary was, in fact, 

 sharply serrate, and at least four nearly parallel north-and-south faults are 

 indicated, three of them also cutting the trap, all with upthrow on the east 

 of the fault. 



Description of the Map. 



On the western side of the map (plate 17), the large number of extensive 

 granite areas will be noted near the valley border in the southern half of the 

 state; and within these limits a vast number of smaller granite dikes occur, 

 which are wanting further northward and westward. The arkose, above 

 four miles wide, adjoins and was derived from this granite from Southwick 

 past Northampton, where it was bored into to a depth of 3,000 feet, and 

 where, at "the oxbow," it is a fine, feldspathic conglomerate. This arkose 



