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BULLETIN OF THE 



also in one exposure distinctly overlies them. This relation enables 

 us to understand how it happened that the coarse quartzose conglomerate 

 is completely surrounded by fclsite which does not appear to have en- 

 tered into its composition. The porphyritic fiilsite being younger than 

 the fragmental rocks, it is not difficult to explain the fact that it contains 

 so many pebbles. In Prospect Hill, Maiden, and the small hill near by, 

 just northeast of the Oak Grove station, the porphyritic felsite appar- 

 ently overlies the fragmental rocks of the same locality. The dark- 

 colored, less porphyritic felsite of the Maiden Highlands is closely related 

 to the porphyritic one to the eastward, and there is good reason to be- 

 lieve that the former, hke the latter, is younger than the tufas of that 

 region. The tufaccous rocks along the Malden-Medford line north of 

 Highland Avenue are composed almost wholly of \orj silicious felsites, 

 unlike those by which the small area is completely surrounded ; a fact 

 which, as it seems to lis, can be explained most satisfactorily by sup- 

 posing that the dark felsite of the Highlands had not been spread upon 

 the surface in its present position at the time the fragmental rocks were 

 formed. The evidence we have given seems to us sufficient to fully 

 cstabhsh the conclusion that some of the felsites are younger than the 

 fragmental rocks with which they are intimately associated ; and on 

 the other hand, it cannot be doubted that these fragmental rocks are 

 more recent than the felsites of whose debris they arc largely composed. 



Diorite. 



The diorite adjoins the felsite for only a few miles along the north- 

 western boundary of the latter, between Melrose Highlands and Smith's 

 Pond (Crystal Lake), Wakefield. The line of contact is generally cov- 

 ered, and the two rocks were not seen upon the same exposure north of 

 West Hill, near the Stoneham station * on the Boston and Maine Hail- 

 road. Upon this interesting hill we find a complex mixture of almost 

 all the rocks of the region, and it appears to us that the diorite pene- 

 trates and envelops not only the stratified group, but also the granite 



I 



and felsite. Distinct fragments of the granite and felsite have been 

 seen in the diorite at several places. Similar phenomena may be seen 

 upon the hills in Stoneham, near the south side of Franklin Street, 

 where the eruptive diorite penetrates the granite, and cuts directly 

 across the bedding of the stratified quartzites and schists. In that region 

 the granites, dioritcs, and rocks of the stratified group are intermingled 



* The name of the poat-otlice -at Stoncliain station is Melrose Highlands. 



