117 



<-the basic series I refer to the probable fact that the granite 

 and petrosilex are unequally metamorphosed portions of one 

 and the same set of beds which were originally of substan- 

 tially similar structure and composition throughout, becoming, 

 however, less acidic upwards ; and to the undeniable fact that 

 the petrosilex or stratified part of this series overlies the gran- 

 ites, just as the so-called stratified group or gneissic part of the 

 basic series overlies the unstratified diorites or " mixed " group. 

 We thus find that all the rocks belonging to the Huronian series 

 in Eastern Massachusetts may be naturally divided into two 

 great groups, which, although defined by their chemical char- 

 acters alone, are petrologically mutually exclusive, or nearly so. 

 Resuming now the inquiry as to the relations of these funda- 

 mental divisions, it can be easily shown that the basic division 

 overlies the acidic. In a region of almost universal north-west- 

 erly dips those strata are probably the newer which lie to the 

 north-west, as does our basic group as a whole with relation to 

 the acidic. I have already noted the stratigraphic continuity of 

 the two series in North Saugus and at other points, where the 

 petrosilex passes insensibly north-westerly into diorite and 

 gneiss. Such localities afford us a clue to the normal arrange- 

 ment : for these gradual transitions between the petrosilex and 

 hornblendic rocks preclude the idea that the former have been 

 thrown down by a fault so as to appear to underlie rocks older 

 than themselves. The granite has been frequently and exten- 

 sively etftravasated through the diorites in some districts ; and 

 the petrosilex, although far less generally eruptive than the 

 granite, also penetrates the basic series. Instances of this on 

 the north-west shore of Marblehead, where the elvanite cuts the 

 diorite, have been described, ante, p. 73. Another example, 

 not before mentioned, occurs on the north-east corner of Marble- 

 head, near Fort Sewall ; here the diorite is cut by a regular 

 dyke of petrosilex, which is nearly black, fine-grained and 

 compact. 1 



1 Since the above was written, I have become aware that Prof. C. H. Hitchcock, in 

 the recently published report on the Geology of New Hampshire, recognizes the fun- 



