166 



between the two divisions being vertical instead of hori- 

 zontal. 



I have already indicated (see under petrosilex) that the term 

 breccia is not descriptive of all the rocks here included under 

 that name, which is employed in a geologic rather than a litho- 

 logic sense, designating a group of rocks, and not a single type 

 only. The fragments in the breccia are often sufficiently 

 rounded to warrant calling them pebbles and the rock lyudding- 

 stone ; so that there is a certain propriety in using the terms 

 fragment and pebble without discrimination, as I have done in 

 the following descriptions. The breccia is everywhere highly 

 feldspathic, being almost entirely composed of the debris of 

 petrosilex ; and the fragments are always firmly imbedded in 

 the paste ; the latter, however, weathers more readily than the 

 former. The intricate and puzzling relations of this breccia 

 group to the Huronian petrosilex have also been sufficiently 

 noticed. Most observers of these rocks have regarded the 

 amygdaloid as an eruptive ; but I am well satisfied that it has 

 played this role only as the exception and not as the rule. 

 Many instances of apparent extravasation are due to faulting ; 

 and I see no room to doubt that this is essentially a stratified 

 rock. Indubitable evidence of the extravasation of the amygda- 

 loid is wanting save where it cuts through the conglomerate, as 

 in Brighton and some other places ; and, since it is certain from 

 the abundant enclosure of amygdaloid pebbles in the conglom- 

 erate, that the former was the surface rock during the deposi- 

 tion of the latter, the inference is plain that, where exotic, the 

 amygdaloid is not far removed from its original seat. As in the 

 case of the breccia, the name of this second division of the Shaw- 

 mut group is little descriptive ; for only a small proportion of 

 the rock here included is a true amygdaloid. 



DETAILS or THE SHAWMUT GROUP. 



Shawmut Group in JSTewbury. — The rocks of this age in 

 the basin of the River Parker belong chiefly to the amygdaloid 

 division. This type is extensively developed over the southern 



