42G 



THE AZOIC SYSTEM AND ITS SaCDlVISIONS. 



extent ; and it is also proved hy the con^^tancy of llielr strike and ilip. It 

 undoubtedly hccomoH more cry.-^tallinCj more granitic, ami glasses into the 

 Quiuey granite, and tlie grauile of Dedliain. It })asses into liiie graim.'d gran- 

 ite toward tlio east and Boutli, Good examples of tlie granitoid, i'elsito, of 



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

 head 



" On, Marhlehcad Neck the breccia, ^yhich 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 iido com- 

 pact felslte, the former in two distinct Avays, which, although observable at 

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

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

 whei-eby the outlines of the former arc lost, or, "when tin; process is not com- 

 plete, can only be seen on weathered surfaces. ('-S) By a Ilatlening or drawing 

 out oi' the pel)l)les into thin lenticular lamina?, wljich, more or less coalescing 

 at their edges and lying in parallel planes, produce a stratihed aii]>earance in 

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



that already descrilj(id, due to original sedimentation It is i'urther 



shown that the massive, structureless felsites, have ])robably been largely 

 derived from massive, obscurely stratified, felds})athic slates, while the nor- 

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



Of the dioritoH 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 (piartz, into fme-grained hornblendic granite ; and it is no less 

 prone, l)y losing hornblende, to pass into felsite. Fine examples of the transi- 

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

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



of hue grained hornblendic granite Any observer of these two rocks 



will agree with me that they admit of neither a litiiological 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 lias been 

 so nvutual, that the comjdlcatiou 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 rooks Lc states (l. c, pp. 23-27) : 



*' As already stated, Ave find, on going northward through Melrose, the por- 

 phyritic felsite, gradually becoming leas porpliyritic and assuming a stratified 

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

 though porpliyritic felsite occurs as far north as Greenwood. North of Central 

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

 northward, are largely quartzose, passing into quartzite, an<l are frequently 

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



