120 



succeeded by the petrosilex of Kent's Island and the River 

 Parker, and this by the eruptive diorites, hornblendic gneiss, 

 and stratified diorite, between the River Parker and the Merri- 

 mac. We cannot, of course, believe that there are here two 

 distinct series of rocks precisely alike lithologically, and pre- 

 senting the same succession. The corresponding members of 

 the two series must be stratigraphically as well as lithologically 

 equivalent ; and, since the terranes are repeated in the same 

 and not in the contrary direction, the repetition is probably 

 to be explained by a fault, arid not by a fold. This grand 

 repetition of the strata seems to necessitate the existence of a 

 gigantic fault extending south-westerly from the shore in Ipswich, 

 and having the down-throw in part, at least, on the south-east. 

 The distribution of the rocks toward the south-west indicates 

 that the fault wanes in that direction, the vertical displacement 

 being insufficient to bring up the granite beyond Middleton ; 

 but it is conceived that the line of fracture extends along the 

 southern border of the Montalban formation through Concord, 

 and does not finally die out before reaching TVestborough ; and 

 it is suggested that we may find in this great dislocation a 

 sufficient cause for the existence of the peculiar Huronian 

 peninsula lying between these towns. This is the grand fault 

 of this region, but many minor fractures exist ; every exotic 

 mass being probably accompanied by a greater or less disloca- 

 tion of the adjacent strata. A short distance south of the 

 Merrimac, in Newburyport and West Newbury, the stratified 

 group is met abrujDtly by granite, which extends northerly 

 over a large part of Salisbury, reaching probably into New 

 Hampshire. At this line of contact a second large fault 

 appears to supervene, inaugurating a second general repetition 

 of the strata. 



It will be observed that between the Rhode Island line 

 on the south, and the New Hampshire line on the north, 

 the Huronian boundary presents two salient angles to the 

 west, one in Concord and one in Groveland. These im- 

 portant deflections or sinuosities are of very similar form, but 



