64 



very rarely exhibits slight traces of banding where a fragment 

 of the original rock has not been entirely crushed, or, more 

 probably, fused. These transformations occur chiefly in the 

 immediate vicinity of the Shawmut breccia, where, as already 

 shown, we have incontestable evidence of the extravasation of 

 portions of the petrosilex. 



The stratified petrosilex of Marblehead Neck, as above 

 stated, is, in the main, less regularly and continuously banded 

 than that in Newbury. The layers are shorter, more lentic- 

 ular, and more prone to coalesce and divide ; i.e., the structure 

 is more schistose and less laminated. The petrosilex of Mar- 

 blehead Neck is very variable in this respect, and it is possible 

 to select a series of specimens which shall exhibit every variety 

 of structure from a regular, agate-like banding to a distinct 

 schistosity where the most of the individual quartzose layers 

 are less than three inches long and decidedly lenticular. The 

 principal members of such a series are shown in PI. 1, figs. 

 1—3, drawn from nature and of natural size ; the shaded por- 

 tions representing the quartzose layers, and the white the 

 feldspathic. Thus I believe we may pass by insensible 

 steps from the most regularly banded petrosilex to the variety 

 represented by fig. 4, also occurring on the Neck. In this 

 there are lighter and darker irregular parallel layers, of varying 

 width, which are characterized by a perfect schistosity, — coarse 

 in the darker bands, but very fine and minute in the lighter. 

 ■There are interspersed crystals of feldspar, but these are neither 

 large nor numerous, and the rock is not distinctly porphyritic. 

 The feldspar crystals, occurring only in the feldspathic layers, 

 could not be shown in the figure. Some of the smaller, dark, 

 lenticular patches have a greenish tinge and are probably 

 epidotic. Such is the weathered aspect ; on a fresh fracture we 

 have simply a black base in which the more prominent struc- 

 ture lines may be dimly discerned. Other parts of the same 

 rock, in some cases of the same hand specimen, have the struc- 

 ture represented in fig. 5. Here we approach the vanishing 

 point of the regular, continuous lamination with which the 



