-The Deerfield Dyke and its Minerals. 195 



Akt. XXII— The Deerfield Dyke and its Minerals ; by Ben. K. 

 Emerson, Professor of Geology in Amherst College. 



Description of the dyke. — The most northern of the large 

 dykes of diabase associated with the Connecticut Biver sand" 

 stone commences in Gill, and after running south westwardly a 

 short distance, swings round to the south and runs down the 

 west side of the Connecticut through Greenfield and Deerfield, 

 and turning eastward crosses the river and ends in Mt. Toby 



i^n \v, 



the elongated L T -shape characteristic of the Tri- 

 dykes of the basin, which appears on a scale so much 

 the Holyoke Range. It is worthy of note that the 

 ern border of the valley corresponds in direction to 

 both these dykes, being set back in Greenfield and Nor' 

 so as in each case to present a reentrant angle to the N.W. 

 corner of the dykes with W.E. and W.S. sides parallel to the 

 corresponding portions of the respective dykes. The dyke is 

 about twenty miles long and at Cheapside, in the north part of 

 Deerfield where the Deerfield Eiver breaks through it, is about 

 30m. thick. The rock is intercalated in the red sandstone and 

 dips eastward with it, but would seem to follow this direction 

 only a little way before coming to the fissure through which it 

 was erupted, as an artesian well sunk on their property in 

 Turner's Falls by the Montague Paper Co. went down in sand- 

 stone 274m. below the level of the Turner's Falls dam, while 

 ■■iy opposite on the west and separated only by the 

 width of the river, about 200m., the trap is about 30m. a'bove 

 the dam, and dips t<>v an! the v ell v ith m angle of 32°. 



Fault at the mouth of Fall Biver.— The best point for the 

 study of this dyke, as indeed for the study of the Connecticut 

 River sandstone in Massachusetts, is at Turner's Falls, and 

 opposite this village at the mouth of Fall River, at the 

 chlorophaeite locality the dyke is beautifully faulted. It 

 comes down from the north to the water's edge, and directly in 

 ■continuation of it in the Connecticut is a sandstone island, 

 while the dyke is offset ten rods to the westward on the other 

 side of Fall River. The proof of the faulting is complete. 

 The sandstones above and below agree bed for bed in all par- 

 ticulars and the outcrops are peculiarly good. This is the 

 only fault in the sandstones which I have been able to prove 

 with certainty. 



The dyke rests to the west on coarse granitic sandstone 

 which it has baked for an inch into a black hornstone and in- 

 fluenced for a foot The diabase is compact in its lower part 

 and amvgdaloidal above, and the soft red shales which rest 

 upon it are wholly unaltered— are never included in the trap— 



