60 B. K. EMERSON — DIABASE BlTCHSTONE AND MUD ENCLOSURES. 



having been cold when they were brought together, have produced no 

 caustic effects upon one another. I know of but one important bed 

 in the Triassic of New England which exactly fits this description. This 

 bed I have called the Granby tuff. Its outcrop starts with the Belcher- 

 town ponds southeast of Amherst and runs parallel with, and a mile 

 south of, the great Holyoke trap ridge, 12? miles to the town of Holyoke. 



In its eastward or shoreward portion it is enclosed in the coarse shore 

 sandstones ; in its central portion it is included in the fine sands of the 

 deep water, and at its south end rests for miles on the upper trap sheet. 



At its bend near the gap where the Connecticut river passes through 

 the trap ridge and near the great Black Rock plug — the largest of the 

 late volcanic cores which cut through the older traps and sandstones — 

 it is made of blocks often one to two feet long. From here the size of 

 the material decreases gradually to the extremities. 



The sandstones beneath it are but slightly ferruginous, and the bed 

 itself graduates upward into a very ferruginous sandstone, and is itself 

 well bedded. It everywhere contains a certain quantity of quartz, mi- 

 crocline, and muscovite, minerals foreign to the trap, but forming the 

 staple sedimentary material of the enclosing sandstones. 



The volcanic material is angular and is composed of the common 

 diabase, sometimes amygdaloidal and sometimes not. It is often much 

 decomposed, and minute fragments can be detected with the microscope 

 in the ferruginous sandstone above into which the tuff graduates after 

 all trace has seemingly disappeared. Under the microscope the altered 

 fragments of the trap rest always against the grains derived from granite 

 without the slightest trace of any reaction between them which could 

 be referred to heat. 



The Black Rock plug is a mile square and is the only core of sufficient 

 size to have furnished by explosion the material contained in the tuff bed, 

 and its position is such as to make it probable that this was its source. 



This is clearly the description of a true bed of tuff. The mass was 

 projected into the air by an explosive eruption, fell into the water and 

 was spread by it as cold material. It graduates into the feldspathic 

 sands which were being spread -over the Triassic basin before and after 

 its advent. 



White Diabase Tuff. 



A smaller bed is very peculiar and deserves a brief description. 



There is a low water-parting between mount Tom and Little mountain 

 to the east, and in the bed of the brook running north, near its source, 

 the sandstone contains a large number of small angular fragments, none 

 more than three inches long, of a white diabase— a rock made up of a 

 mass of plagioclase rods, with an ophitic structure like the common 



