94 



inferior hardness and more waxy lustre. In the Lynn and 

 Medford area the felsite appears to lie mainly to the north of 

 the petrosilex, and, since the prevailing dip here is north-west- 

 erly, the former rock probably overlies the latter ; this, I 

 believe, is their normal relation. 



That the banded petrosilex is stratified may be regarded as 

 fully proved ; and that the schistosity of the schistose rock, 

 although, perhaps, not the direct result of sedimentation, cor- 

 responds to or represents genuine stratification, i.e., is some 

 modification of an originally sedimentary structure, is almost 

 equally certain. Now, since there is not a considerable mass 

 of petrosilex in this region in which examination will not dis- 

 close traces at least of one or other of these structures, I think 

 the inference is legitimate that all our petrosilicious rocks are 

 of aqueous origin. Yet it is unquestionable that, in its pres- 

 ent condition, some of the petrosilex is exotic. 



The great mass of the rocks here described under the head 

 of petrosilex is undoubtedly Huronian ; but it is by no means 

 certain that this system will include them all ; for, as already 

 hinted, indications of a difference of age are not wholly want- 

 ing, especially among the felsites. It seems best, however, in 

 the absence of satisfactory evidence to the contrary, to continue 

 to refer the whole to the same horizon. If proved to belong 

 to distinct epochs, I should assign the more ancient to the 

 Huronian system. 



. In its general distribution the petrosUex obeys the same law 

 as the granite. The areas usually have trends approximating 

 N.E.-S.W., and are so disposed as to form ranges or belts 

 parallel with the ranges of granite. The Newbury area is one 

 of the simplest of these belts, and it conforms perfectly with 

 the trend of the enclosing granite. Our longest line of granite, 

 the Natick and Rockport belt, is bordered on the south by the 

 longest range of petrosilex. This begins as a narrow band 

 skirting the granite in South Natick and northern Needham, 

 and is probably continuous, under the conglomerate and slate 

 in Newton, Watertown, Belmont, and Arlington, with the 



