426 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



extent ; and it is also prov^ed by the constancy of their strike and dip. It 

 undoubtedly becomes more crystalline, more granitic, and passes into the 

 Quincy granite, and the granite of Dedhani. It passes into fine grained gran- 

 ite toward the east and south. Good examples of the granitoid felsite, of 

 small extent, occur on Lowell's Island and the north-west shore of Marble- 

 head 



" On Marblehead Neck the breccia, which is here more properly a conglom- 

 erate, becomes at some points, especially on Lowell's Island, a coarse, gritty, 

 feldspathic sandstone, and both the conglomerate and sandstone pass into com- 

 pact felsite, the former in two distinct ways, which, although observable at 

 several points in this region, are best exemplified here. These two modes of 

 metamorphism are : (1) By a blending together of the pebbles and paste, 

 whereby the outlines of the former are lost, or, when the process is not com- 

 plete, can only be seen on weathered surfaces. (2) By a flattening or drawing 

 out of the pebbles into thin lenticular laminae, which, more or less coalescing 

 at their edges and lying in parallel planes, produce a stratified appearance in 

 the rock, and give rise to a laminated or banded structure closely resembling 



that already described, due to original sedimentation It is further 



shown that the massive, structureless felsites, have probably been largely 

 derived from massive, obscurely stratified, feldspathic slates, while the nor- 

 mally banded felsite represents a finely and distinctly stratified slate." 



Of the diorites he remarks (Report, pp. 22, 23) : — 



" The diorite, like the granite, varies greatly in texture and composition. 

 .... In composition it has a wider range ; as already stated, it passes, by an 

 admixture of quartz, into fine-grained hornblendic granite ; and it is no less 

 prone, by losing hornblende, to pass into felsite. Fine examples of the transi- 

 tion between diorite and felsite may be seen in Greenwood and Stoneham. 

 .... The areas colored as diorite on the map .... embrace a great amount 



of fine grained hornblendic granite Any observer of these two rocks 



will agree with me that they admit of neither a lithological nor a geographical 

 separation. As a rule they are both eruptive, and over large areas they have 

 been extravasated through each other so extensively, and the action has been 

 so mutual, that the complication is complete ; and I have long been accus- 

 tomed to speak of them as ' mixed rocks ' ; and I know of no term that will 

 better express their relations, lithologically or petrologically." 



Of the stratified rocks he states {I. c, pp. 23-27) : — 

 " As already stated, we find, on going northward through ^Melrose, the por- 

 phyi-itic felsite, gradually becoming less porphyritic and assuming a stratified 

 appearance. North of Howard Street traces of stratification are common, 

 though porph}Titic felsite occurs as far north as Greenwood. North of Central 

 Brook, in Saugus, the felsites are chiefly stratified, the bedding increasing 

 northward, are largely quartzose, passing into quartzite, and are frequently 

 interstratified with hornblende slate and stratified diorite. No observer, who 



