260 



Salem Street, and between Woburn and Purchase Streets. The 

 fine-grained, euritic granite is extensively developed in the north- 

 west part of Medford, and forms a prominent ledge on Pur- 

 chase Street about one-fourth of a mile north-west of Salem 

 Street. Going south from this we come first to outcrops of a 

 hard, compact, felsitic rock, which is not distinctly stratified, 

 and which closely resembles the rock that comes between the 

 granite and conglomerate on the northern flank of the Blue 

 Hills. 



South of this felsite, or felsitic quartzite, but not seen in 

 contact with it, comes the first of the conglomerate. This is 

 mostly small-pebbled and slaty, containing some pinite, and 

 showing a southerly dip of about forty-five degrees. The con- 

 glomerate has a breadth of at least three hundred or four hun- 

 dred feet, and is represented by several ledges. It is inter- 

 stratified with, and seems to pass gradually into, a firm, fine- 

 grained, grayish to whitish sandstone, which is mainly a true 

 quartzite, though sometimes feldspathic. This resembles the 

 rock north of the conglomerate, but is, I think, more distinctly 

 arenaceous and more evidently stratified. It certainly over- 

 lies the conglomerate ; and yet I suspect that it may be con- 

 tinuous, across Salem Street, with the compact felsitic rock 

 which I have marked on the map as forming a wedge-shaped 

 area tapering toward the west, and referred to the Shawmut 

 group. If this point were established, then the felsitic rock, 

 which outcrops very prominently between Salem Street and the 

 Mystic, would appear to be the equivalent of the slate for- 

 mation. 



Considered as a whole this section from the Medford granite 

 southward is substantially similar to that on the opposite side 

 of the basin from the Blue Hill granite northward, as exposed 

 in the vicinity of Randolph Turnpike and farther west. The 

 position of the conglomerate in Medford is an additional and 

 plain confirmation of the statement on a preceding page that 

 the outcrops of this rock occur normally between the crystallines 

 and the slate along the margin of the basin. 



