GEOLOGY AND MINERALOGY OF NEWBURY. 165 



The term porphyry has cleservedly fallen iiito disuse as 

 a Substantive, and is now rightly used as an adjeetive. 

 All the varieties once called porphyry are now arranged 

 as varieties linder the several great species or families of 

 rocks. 



The term felsite is used to include rocks composed 

 mainly of a fine paste of quartz and feldspar, sometimes 

 enclosing grains of quartz and crystals of hornblende and 

 feldspar, sometimes banded like Jasper, and sometimes 

 segregated like conglomerate. Examples of the compact 

 varieties are the so-called Jasper of Saugus and Lynn ; 

 the banded varieties are found at Marblehead, and an 

 example of the segregated variety is the so-called toad- 

 stone of Newbury. 



The Newbury felsite extends in a belt along the River 

 Parker, from its mouthto Byfield, a distance of five miles. 

 Its width is from a few rods to a mile and a half, and 

 lies between belts of granite. 



Its prevailing color is a deep red or brownish red, but 

 sometimes shades to purple, pink and gray. It is never 

 porphyritic, but shows a banded structure, due to the inter- 

 lamination of layers of quartzose and feldspathic materials. 

 This banding indicates a sedimentary, not an igneous ori- 

 gin, though like all metamorphic rocks, our felsite has 

 undergone material change. 



I have already alluded to the so-called toadstone of 

 Newbury as a variety of felsite. A füll examination of 

 this rock will be found in Crosby's Geology of Eastern 

 Massachusetts, which I have largely followed in this 

 paper. 



In Newbury the felsite is highly ferruginous. The sides 

 of the granitic basin in which it lies partake of the red- 

 dish cast of the felsite. In some places it might not im- 



ESSEX INST. BULLETIN, VOL. XVI. 11* 



