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The Newer Eruptive Rocks. 
BY GEORGE P. MERRILL. 
The more recent eruptive rocks of Nantasket, including 
all those of later date than the diorite, granite and felsite, 
oceur in the form of (1) lava-flows and (2) dikes. "The lava- 
flows include both melaphyrs and porphyrites and these vol- 
canic types are also found among the dikes, although the 
latter consist chiefly of diabase. The melaphyrs and dinbases 
have, in many cases, undergone such complete alteration that 
their correct identification is attended with the greatest difficulty. 
Indeed, the slide often shows not a single recognizable original 
constituent; but consists wholly of a compact agereg: 
secondary minerals, among which are epidote, quartz, kaolin, 
and sundry chloritie and ferruginous products for which there 
are, perhaps, no better names than the terms viridite and 
opacite, already in general use. 
Perfectly satisfactory results from such materials could be 
obtained only by prolonged and careful microscopie examina- 
tions and chemical tests, accompanied by study in the field, 
where the rocks could be observed in all their varying aspects 
and in varying stages of alteration. Such study I have natur- 
ally been unable to give them, and my main effort has been to 
80 identify the rocks and note their essential differences, if any, 
48 to aid Professor Crosby in his difficult task of ascertaining 
their field relations. 
Tum Lava-rLows om VOLCANIC Rocks. 
The effusive rocks or true lavas of Nantasket embrace, as 
Stated, two principal types — melaphyr and porphyrite. The 
melaphyr is the predominant type and forms several distinet 
flows or sheets, of varying lithological character ; while the por- 
phyrite, 80 far as known, may all be referred to one widely 
extended sheet. U [ 
ate of 
