WARREN. — ALKALI-GRANITES AND PORPHYRIES. 293 



a thin skin about and through which the underlying igneous rocks 

 appear constantly. Against the slate, the normal development is 

 first the relatively basic feldspar-porphyry. This is succeeded by a 

 thin zone of granite-porphyry passing into the porphyritic coarse- 

 granite. In places, fine-granite appears to come in as a contact phase 

 as noted earlier. The coarser granite intrudes the fine-granite and both 

 porphyries in many small irregular dikes, and in the extreme western 

 edge of the area, running for about one half of its length (in part out- 

 side the northern boundary of the reservation) is a large dike of variable 

 width and quite irregular trend which has a total length of something 

 over a thousand feet. The width A'aries from a few feet to as much as 

 100 at the widest part. Just north of the boundary fence this dike 

 is well exposed for study. Its eastern side, westward from the slate 

 for perhaps ten feet (exposed), consists of a fine-granite-porphyry 

 containing fairly abvmdant phenocrysts of feldspar and qviartz and 

 abundant specky hornblende. The center consists of the porphyritic 

 type of coarser granite in which are embedded a great number of 

 inclusions of the basic feldspar-porphyry of the same composition and 

 character as that found in the massive ledges only a little distance 

 away where the slate cover has been worn away. These inclusions 

 are of all sizes from one-half centimeter to those which will measure 

 two feet across, though the average will not probably exceed seven or 

 eight inches. In shape they are sub-angular, rounded, elliptical or 

 irregular, closely similar in fact to the inclusions described as occurring 

 in the granite of the glaciated ledge of the Pine Hill area, West Quincy 

 (p. 274). Some slate inclusions are also present. Though the west- 

 ern side of the dike is not well exposed, the fine-granite-porphyry 

 appears to form the border of the western contact also. The dike is 

 doubtless a very shallow one, hardly more than an upward protuber- 

 ance of the invading igneous mass into the slate cover. The composi- 

 tion of this dike is satisfactorily explained if we assume that prior to 

 the intrusion of the dike, the magma beneath the slate had differen- 

 tiated and partly crystallized with the development of the rhomben- 

 porphyry next to the slate. Pressure from below then caused the 

 magma to burst through the cover of slate above, breaking up the 

 layer of basic feldspar-porphyry and carrying its fragments, together 

 with slate, up into the dike channel mingled with the granitic magma. 

 Toward the margin, the dike assumed a finer grained and porphyritic 

 texture, but centrally crystallized as granite. 



Small dikes and patches of pegmatitic character always containing 

 some fine-granite material *^ are found; one on the Rattle Rock; 



47 See Warren & Palache, loc. cit., p. 127. 



