436 



THE AZOIC STSTEM AND ITS SUBDIVISIONS. 



deposition differed iu degree only, if at all, from those that have obtained in 

 more recent geological times So far as I am uware, those liinited por- 

 tions of the Huronian petrosilex containing distinctly marked pebbles occur 

 chiefly in the immediate vicinity of the petrosilex breccia of the Sliawmut 

 group ; and in nearly every instance tlie evidence is plain that the petrosilex 

 has experienced some extravasation, and has, consec^uently, been in a more or 



less fluent state On the eastern shore of Marblehead JSTeck .... the 



former reposes directly upon the latter ; and both series have evidently suf- 

 fered great disturbance. The light-colored Hhawmut breccia is cut and torn 

 in every direction by tortuous dykes of the black petrosilex, which itself veiy 



clearly holds angular pebbles of different varieties of petrosilex Tlie 



main point to be proved in this connection is brought out at this locality with 

 especial clearness; viz., that the very same petrosilex which holds pebbles 

 underlies, and is in i)art eruptive through, the breccia, forcing tlie conclusion 

 that, in spite of a certain superficial resemblance, these are distinct formations. 

 .... To summarize, the facts oliservable at the places named, and elsewhere 

 compel us to suppose that the petrosilex, while suffering great disturbance has 

 been locally crushed and brecciated, and that certain portions of this rock per- 

 haps as a consecpicnce of enormouB friction, have been softened to an extent 



that would i)ermit the envelopment of extraneous masses The tlieory 



of the conglomerate origin of the banded petrosilex .... is I think based 

 upon an entire misapprehension of the pnncipal facts." 



He admits the extravasation of the fclsitc through the brecciutcd 

 material or ash, and states that the evidence that the banding was 

 formed by flattened pebbles " is wholly illusory." (I. c, p. G8.) 



He claims that in Melrose is a locality that 



"places beyond question the fact that there is a gradual transition between the 

 quartzite and petrosilex, and that portions of tlie latter rock are hiterculated 

 in the stratified group." (/. c, p. 106.) 



This locality, however, Avas, as already mentioned, studied by Mr. 

 Diller, and nothing of the kind found there. Dr. Wadsworth also ex- 

 amined the reeks in place at that locality, and found no felsite or any 

 other rock that ought to be mistaken for a felsite. 



Mr. Crosby further states : — 



" The conclusion is now certainly safe that the eruptive dioritcKS and the 

 stratified group are unequally metamorphosed portions of one great series of 

 basic rof^ks." {I. c, p. 112.) 



But this, like his other conclusions, rests on his inability to distin- 

 guish between different rocks, and is purely theoretical. 



In 1870 Prof. N. S. Shalcr took the around that the shales and con- 



glomerates of lioxbury passed into the amygdaloidal mekphyr, locally 



