LITHOLOGY. I TJ 



PORPIIYRITIC FeLSITE (PoRPIIYRIES). 



The porphyritic felsites, or porphyries, constitute a large family of 

 rocks in New Hampshire, and the great variety in their structure and 

 mode of occurrence makes the study of them very interesting. At times, 

 like the felsites, they fill small dykes, but often they form mountain 

 masses. Before proceeding to their description, it will be well to call 

 attention to studies that have been made elsewhere in our region. 



The porphyries are usually massive eruptive rocks. They form im- 

 mense dykes ; and in Silesia the columnar structure, and all other pecu- 

 liarities of ordinary igneous rocks, are often seen. Dr. Hunt describes 

 the porphyries of Canada as forming dykes, and in New Hampshire most 

 of our porphyries are plainly enough eruptive. But in Massachusetts, in 

 the neighborhood of Boston, there is a grand display of porphyries, fel- 

 sites, etc.; and Mr. T. T. Bouve* has shown that every shade of varia- 

 tion between most compact porphyry and conglomerate is to be there 

 seen. This observation, which has been confirmed by other observers in 

 other places, led Mr. Bouve to assert the origin of porphyries from sedi- 

 mentary deposits. Mr. J. C. Ward has also pointed out the passage by 

 insensible gradations of certain fragmentary rocks into quartz porphyries 

 in England.! If, now, the views in regard to the re-fusion of sediments, 

 which have already been explained, are true, then the observations made 

 by Mr. Bouve are what would be expected, as a result either of a variation 

 in the efiiciency of the causes producing the fusion, or in the condition 

 of the sediments submitted to their action. The same circumstance will 

 explain the existence of many porphyries in New Hampshire, as, for ex- 

 ample, a variety at Newcastle, and others elsewhere, which resemble 

 sandstones while they have the features of porphyries, and which, even 

 when microscopically examined, are hard to name. It is not our inten- 

 tion to describe in detail these peculiar or doubtful varieties, but, recog- 

 nizing their existence, and the light which they throw on the general 

 subject, we propose to devote the space to the description of the inter- 

 esting features presented by those immense masses of typical porphyry 

 which are so important to our lithology. 



* Proceedings Boston Society of Natural History, 1862, p. 57; 1876, p. 217. 

 t Quarterly Journal Gcol. Soc, No. izs, p. 25. 

 VOL. IV. 23. 



