THE LYMAN SCHISTS OF NEW HAMPSHIRE 377 



contain inclusions of the groundmass. This rock is remarkably 

 like a pebble taken from a conglomerate which Hitchcock 1 called 

 the "egg conglomerate" (see Fig. 15), a pebble which would be 

 called a felsitic rock without hesitation. 



GENERAL CONCLUSIONS ON THE ORIGIN OF THE LYMAN SCHISTS 



Among the characters described in the foregoing paragraphs 

 many are of such a nature as to suggest that at least some members 

 of the Lyman schist group are igneous rocks — probably volcanic — 

 more or less modified by dynamic metamorphism. For the sake 

 of clearness and emphasis, the significant facts which lead to this 

 conclusion are reviewed below in summary form for three typical 

 rocks. 



Porphyry schist of the Young's Pond locality. — The quartz- 

 plagioclase porphyry schist which has been described from the 

 Young's Pond locality is assumed to have been an effusive rock 

 for the following reasons: 



1. It has a strong resemblance to the quartz porphyry type of 

 igneous rock. 



2. It is massive, without bedding. 



3. Locally it has faint indications of flow structure. 



4. The phenocrysts are rather uniformly distributed through 

 the groundmass. 



5. The quartz phenocrysts have high luster and high trans- 

 parency, a faint bluish opalescence, and a tendency toward 

 cleavage, all these being features not uncommon in true quartz 

 porphyries. 



6. Some of the feldspar phenocrysts have crystal form. 



7. There is a great difference between the size of the smaller 

 phenocrysts and that of the larger groundmass particles. 



8. Many quartz phenocrysts and some feldspar phenocrysts 

 contain embayments of the groundmass, these embayments having 



1 C. H. Hitchcock, Geology of New Hampshire (1877), II, 333. This "egg con- 

 glomerate" is exposed on the northwest slope of Blueberry Mountain and may grade 

 laterally into the Fitch Hill arkose. Most of its pebbles are of quartz porphyry, 

 quartz keratophyre, granophyre, devitrified rhyolite, and other felsitic types. The 

 great preponderance of effusive rocks among the pebbles is to be noted for comparison 

 with the Lyman series. 



