. 65 



series began ; and from this rock the passage is natural and 

 easy to one in which that structure is almost wholly wanting. 

 Such a type exists on Marblehead Neck, and is represented by 

 fig. 6. Compared with the first member of the series it may 

 be regarded as a distinct variety of petrosilex. This rock 

 occurs at several points on the Neck, but has its greatest de- 

 velopment at Castle Rock on the eastern shore, where it is 

 associated with the types represented in figs. 4 and 5. Like 

 these, it shows on a fresh fracture a compact black base, with 

 faint traces of lighter and darker stripes ; and on the weathered 

 surface the structure comes out prominently, revealing numerous 

 elongated, more or less parallel, usually lenticular, black, 

 brownish-black or ash-colored layers, of all lengths up to three 

 inches, and varying in width from a mere line to one-fourth of 

 an inch. As shown in the figure, the smaller layers predomi- 

 nate, the rock being finely schistose, with now and then a coarse 

 layer. The outlines of the layers are not smooth and regular ; 

 on the contrary, they are exceedingly broken and uneven ; 

 being frequently curved and ending abruptly, or the edges are 

 bifurcated or split up irregularly into several prolongations of 

 varying thickness, but often of extreme tenuity. Many of the 

 larger layers are themselves schistose. The brownish and ash- 

 colored patches have a granular texture, are less firm than the 

 rest of the rock, and some of them appear to be epidotic. The 

 strike of this rock, although very variable, averages north- 

 east. The parallel arrangement of the layers and the general 

 evenness of the structure have been much disturbed in a small 

 way, which is not apparent at first view. It is as if the rock 

 had been wrenched or shaken in such manner as to produce 

 numerous small dislocations, contortions, and local crushings. 

 The structure of some portions of this rock, in the vicinity of 

 Castle Rock, is still farther complicated by the presence among 

 the layers of distinct pebbles. These are of all sizes up to two 

 inches in diameter, and their outlines are sharp and angular. 

 Occasionally they are of the same character as the enclosing 

 rock, and may then be explained by the theory of internal frac- 



OCCAS. PAPERS B. S. N. H. — III. 5 



