57 



trend of the belt ; the corresponding dip is to the N.W. and 

 N., and usually quite steep. 



This banding or lamination is frequently met with in the 

 petrosilex and felsite of other regions, and is familiar to all 

 geologists. In England and on the continent of Europe, where 

 it is the universal belief that petrosilex and felsite are always 

 and everywhere of igneous origin, where they are regarded as 

 trappean rocks, and so classified by systematic lithologists, this 

 banded structure is naturally and commonly interpreted as 

 probably resulting, like the variously hued lines and bands in a 

 slag from an iron furnace, from the motion of the mass when in a 

 pasty and semifluid condition. In this country radically differ- 

 ent views are gaining ground. Prof. Dana, in his "Manual 

 of Geology," classes the petrosilicious group as metamorphic 

 rocks ; and Dr. Hunt has long taught their aqueous or sedi- 

 mentary origin. This view is the only one in harmony with 

 the facts observed in this region, and especially with the band- 

 ing, or, as I prefer to call it, stratification, of the petrosilex. 

 The bands in question are undoubtedly lines of bedding ; and 

 the lamination has substantially the same signification in petro- 

 silex as in slate or sandstone. The petrology of the banded 

 petrosilex at some points in Eastern Massachusetts yet to be 

 described, absolutely forbids the acceptance of the motion-in-a- 

 plastic-mass theory of the origin of the banded structure. 



The Newbury rock is, for the most part at least, a genuine 

 petrosilex. A typical specimen of the banded variety from 

 Kent's Island contains 75.7 per cent, of silica. This indicates 

 at least 25 per cent, of free quartz in the rock. Another speci- 

 men from a point further west, near Dummer Academy, afforded 

 76.4 per cent, of silica. 



Concretionary structure. — South of the River Parker and 

 near the road leading south from Old Town, at a place known 

 as the Bartlett Mine, is a rock with a singular concretionary 

 structure, which the miners call "toad stone." It presents a 

 spotted appearance, whence the name. This rock must, I 

 think, be regarded as a variety of the petrosilex ; in fact, a 



