Veins in Mica Slate. 



341 



cester county, where the strata usually run north and south: while 

 the great body of this range is connected with the systenf of stratifica- 

 tion that prevails in the gneiss range, running northeast and south- 

 west, in the east part of Worcester county, and the west part of Mid- 

 dlesex. South of Worcester the western margin of the gneiss lying 

 east of the slate, runs nearly north and south; and hence the mica 

 slate there affects the same direction, except in the vicinity of the an- 

 thracite bed; where it curves around the north end of the hill of 

 granite, west of the mine. As we proceed northerly, the gneiss trends 

 away more towards the northeast, and the mica slate conforms to it. 

 Still farther to the northeast, the strata of the mica slate turn more 

 easterly ; because the longer axis of the valley of the Merrimack lies 

 in that direction ; and in this the slate seems to have been originally 

 deposited. 



These suggestions may explain some of the irregularities appar- 

 rent in the stratification of this mica slate. And when we recollect, 

 that numerous masses of granite are protruded through it — some of 

 them of great extent, as at Fitchburg, for example, — I think we shall 

 have no difficulty in explaning the remaining anomalies. 



The strata of mica slate in Sherburne, run northwest and south- 

 east, and dip northeast about 45°. This small deposit is very obvi- 

 ously connected with that system of strata which appears in the 

 gneiss of the southeast part of Worcester county ; as will be manifest 

 when I come to describe that rock. 



At Woonsocket Falls in Cumberland, R. I. a peculiar mica slate 

 appears running south several degrees west, and dipping southeast 

 Trom 60° to 80°. 



Veins in Mica Slate. 



These consist chiefly of granite and quartz ; but it will be more 

 convenient to describe them when treating of granite. 



Some of the more close grained and imperfectly schistose varie- 

 ties of mica slate exhibit by disintegration, that kind of structure 

 which has been sometimes denominated veins of segregation: that is, 

 veins produced at the time of the formation of the rock, or when 

 it was in a fluid state, by the play of chemical affinities, which 

 in a measure separated the ingredients into different masses ; so that 

 when atmospheric agencies wear away the rock, the harder parts re- 

 main in relief on the surface, like genuine veins. The following is a 

 sketch of a bowlder of mica slate not more than two feet in diameter, 



