83 



porphyritic, but banding is not uncommon. Only a few of 

 these limited areas are shown on the map. 



The most interesting, and at the same time the most beau- 

 tiful, petrosilicious rock in this Hyde Park region is the 

 schistose or striped variety. The best exposure of this is 

 near the Neponset River, in Hyde Park Village, at the inter- 

 section of River and Arlington Streets. The general plan of 

 the structure is the same as in the " flattened pebble " petro- 

 silex on Marblehead Neck. The rock is mainly of two va- 

 rieties, red and gray, and these are quite distinct; the former 

 predominating. The red variety consists of a reddish and 

 sometimes pinkish base, with irregular lenticular layers of a 

 darker red; these are of all sizes, up to an inch thick, 

 and nearly a foot in length ; as a rule, however, they are not 

 more than one-fourth of an inch thick, and four to six inches 

 long. It will be noticed that the layers are longer than 

 in the Marblehead rock ; and they are also more numerous, 

 the base being relatively less abundant. The very fine schistos- 

 ity, though not wholly wanting, does not characterize the 

 rock to the extent observed in other places. On the fresh sur- 

 face the lenticular layers are little conspicuous ; but weather- 

 ing brings them out distinctly. They are more quartzose 

 than the base, which is usually soft and whitish where 

 exposed. 



Other layers or patches, still more irregular, and much fewer 

 than those described, are bright red or light pink, have a jas- 

 pery appearance, and are wholly unaffected by weathering. 

 These jaspery patches sometimes form thin sheets, similar to 

 and parallel with the other and more feldspathic layers. (PL 

 2, fig. 2.) As a rule, however, they have the general shape 

 of angular pebbles ; and yet they certainly are not pebbles, for 

 in nearly every case one or more of the angles are drawn out 

 into thin, sharp-edged, curving layers or strings, which would 

 be extremely unlikely to result from any conceivable mode of 

 fracture, and which, on the other hand, cannot be appealed to 

 as evidence of softening and flattening, since the general out- 



