WARREN. — ALKALI-GRANITES AND PORPHYRIES. 259 



passes eventually into the normal Quincy granite. The width of the 

 zone measured normally, from the contact to the point, or rather 

 interval, in which the rock can be fairly termed a granite, in some places 

 is certainly not over 10-15 feet while in others, less easily determined, 

 it is probably in the neighborhood of 30-40 ft. Hoav far it is to where 

 the normal Quincy granite is typically developed can not be estimated 

 exactly, but it is safe to say that it is measured by not over a few tens 

 of feet in this part of the area. 



After passing the summit of Pine Hill the character of the contact 

 porphyry changes (fault?) and it is less favorably exposed for study. 

 It appears to pass into the Rattlesnake Hill type but as the extreme 

 western end of the area is reached there is again a change, fine granite 

 and more basic phase of the porphyry coming in, which are indicative 

 of a near approach to a slate contact, such as in fact is actually ex- 

 posed in tlie northern part of the area as shown on the special map. 



On the southern and sharply rounded face a small hill to the east of 

 the railroad track running east of the Pine Hill area, is a thin cover of 

 granite-porphyry. This lies on the granite and forms a sharp contact, 

 marked by the development of ainmdant long hornblende crystals, 

 which dips a rather low angle to the south. Immediately beneath 

 this porphyry numerous riebeckite pegmatitic dikes and stringers 

 occur in the granite. ^^ This porphyry is unusual in its texture. Over 

 small irregular areas it is strongly and coarsely porphyritic (feldspar 

 with subordinate quartz) otherwise it is rather feebly and unevenly 

 porphyritic. The microscope shows, that the feldspar, phenocrysts 

 and groundmass, is a microperthite like that of the granite but con- 

 tains a greater proportion of albite, and the latter is very strongly de- 

 veloped marginally and as distinct crystals. The quartz tends toward 

 micrographic intergrowths and its total amount is relatively high. 

 Aegirite is abundant alone and intergrown with the riebeckite, which 

 here has often an unusually great elongation; accessories as in the 

 granite. It has possibly been affected by pneumatolitic action con- 

 nected with the pegmatitic intrusion, but in any case it shows a varia- 

 tion from the other types of granite-porphyry that is of interest. Its 

 feebly porphyritic phase resembles closely the more acid type of 

 xenoliths. 



So far the characteristics of the porphyry with relation to its 

 contact with the aporhyolite have been considered, \yhat its con- 

 tact phases were with relation to the higher slate and intermediate 



38 Warren & Palache, op. eit., p. 



