239 



slate is very thin and even. The slate is gray, or, in part, of 

 a purplish-brown color, and somewhat contorted. It reaches 

 about half way up the slope above the road, and is then over- 

 laid by conglomerate, which extends, without sensible inter- 

 ruption, to the great mass of amygdaloid on the* north, a 

 distance of half a mile. 



All the stratified rocks in this section have a northerly dip 

 of 30° to 40°, and from the slate downwards they appear 

 entirely conformable. The slate and the overlying conglom- 

 erate, however, are clearly unconformable. In the first place 

 the passage is perfectly abrupt, no shading of the one rock 

 into the other ; and the conglomerate cuts off the slate very 

 obliquely, both in the direction of the dip, i.e., the conglom- 

 erate has a steeper dip than the slate, and along the strike, so 

 that the conglomerate gradually approaches the street as we 

 follow the contact east or west from the highest layer of slate. 

 In fact, the portion of the slate exposed here is almost as 

 limited along the strike as across it, being entirely cut off by 

 the conglomerate at a distance of a few rods towards the east, 

 and the same appears to be true on the west ; but in this di- 

 rection the unconformity might result in part from a transverse 

 fault. The unconformajbility of the slate and conglomerate is 

 proved, also, by the aqueduct tunnel, which has a course almost 

 exactly parallel with the strike of the beds ; it begins on the 

 west in conglomerate, but soon passes into the slate, which it 

 follows for perhaps one hundred and fifty feet, and then 

 re-enters the conglomerate, both transits being entirely abrupt. 



I have searched in vain for croppings of slate between this 

 place and the reservoir ; but when drawing the map I con- 

 sidered the slate as continuous between these points, though 

 concealed by the drift ; it now seems probable, however, that 

 this view will have to be abandoned, for a portion of the 

 distance at least, since over much of the intervening ground the 

 conglomerate ledges are so numerous as to practically leave no 

 room for any considerable bed of slate. 



West of Newton Centre the slate appears again in an obscure 



