GEOLOGY OF THE CONNECTICUT VALLEY DISTRICT. 337 



are traversed by beautiful ribbons of banded trap, from half an inch to 

 three inches in width, which jump and curve in a fanciful way. 



The next series evidently belongs to the Lisbon group, having essen- 

 tially the same strike and dip with the last mentioned beds. It extends 

 for about twenty-five rods along the stream. The rock is a labradorite- 

 diorite, according to Mr. Hawes's careful determination. Probably this 

 is the place for a coarse hornblende aggregate or diorite rock, which is 

 seen on the west side of Streeter pond in situ, and in enormous boulders 

 between North Lisbon and the pond. These hard rocks terminate just 

 as the stream bends and passes through a narrow, rocky gorge of clay 

 slates, containing staurolite and garnet, dipping 60'' N. 2cf W. The 

 same dip is manifest higher up. The ledges extend for twenty-five or 

 thirty rods ; after which the rocks are covered with earth for a great dis- 

 tance. The banks are made up of boulder clay, which is undermined by 

 high freshets ; and the road has been torn away so many times that the 

 town authorities have been compelled to discontinue the carriage-road, 

 thus necessitating pedestrian explorations in this interesting spot. A 

 slate similar to that just described crops out on the new road from the 

 station to Streeter pond, about a mile north-easterly. The course is 

 nearly east and west, and the strata are more nearly vertical; but the 

 ledges must be continuous between these points. It is shown, also, by 

 the presence of enormous blocks of diorite, whose source must be to the 

 north of the slates, and not far distant. The map shows the distribution 

 of all the parts of this section. The width of the coarse conglomerates 

 is about forty rods, which implies a thickness of at least five hundred 

 feet. The clay slates correspond perfectly to portions of the Coos group 

 in the Lyme and Lisbon sections, as well as to the interesting staurolite 

 slate seen at Purple's quarry in Bernardston, Mass., described by Prof. 

 Dana.* 



When I first made this section (Fig. 44) I believed the coarse ''o-g^'^ 

 conglomerate of Blueberry mountain and the accompanying sandstones 

 and slates belonged to the Helderberg. Hence the discovery of isolated 

 beds at North Lisbon led to the supposition of the existence of a Helder- 

 berg area there. The chief evidence of this Helderberg age, however, I 



* Amer. your. Sci., Ill, vol. vi, p. 348. 

 VOL. IT. 43 



