WARREN. — ALKALI-GRANITES AND PORPHYRIES. 253 



because at times its grains appear to have suffered some enlargement. 

 It thus happens, as a result of the changes enumerated, that the 

 granite-porphyry is not infrec^uently reduced to a fine grained aggre- 

 gate which shows only a faint indication of its originally richly 

 porphyritic character. In such, where superficial weathering has de- 

 veloped a large amount of hematite, etc., the rock has the appearance 

 of a feebly porphyritic, reddish or purplish felsite and may not at first 

 sight be easily distinguished from the aporhyolite of this area. 



Microscopic Characters of the Porphyry nearer the Contact. — As 

 noted under the megascopic description, the porphyry undergoes a 

 marked change in texture whenever its contact with the aporhyolite 

 is approached, and although with the exception of the two special 

 areas, Pine Hill and the Pine Tree Brook Reservation, no actual 

 contacts with any other rock occur, there are considerable areas over 

 which the porphyry shows, both in the hand-specimen and under the 

 microscope, unmistakable evidences of being a contact phase. 



Thin sections cut from several series of specimens, taken every few 

 inches from the contact going toward the aporhyolite, show that the 

 first change that becomes apparent is in the hornblende groups. They 

 are smaller, relatively more numerous and tend toward an imperfect, 

 short prismatic habit. While still intergrown with the groundmass 

 grains they commonly show a tendency to grow about one or more of 

 the groundmass grains as a center. The massive crystals are less 

 numerous. The aegirite of the groundmass assumes a distinctly 

 more prismoid habit, the crystals being short, stout and better formed, 

 and they often show a tendency to form clusters and to a parallel 

 arrangement. The larger aegirites have much the same habit as 

 elsewhere in the porphyry. 



As the contact is approached more closely there is a rather sudden 

 change in the groundmass. It becomes much finer, the aegirite forms 

 very abundant minute prismoids which are commonly arranged with 

 a distinct flow structure about the phenocrysts. The riebeckite also 

 forms minute rods and flakes with a somewhat elongated habit. 

 Minute magnetite octahedra and hematite grains are abundant. 

 The phenocrysts of feldspar and quartz are somewhat less numerous 

 and the quartz has increased relatively to the feldspar (as shown 

 by the Rosival measurements in the table, p. 243, Column II). 

 Though frec^uently imperfect, particularly on one or two sides, and 

 often broken, the feldspar crystals appear to have been well developed 

 crystallographically as do also the quartz phenocrysts, the latter 

 often showing a quite perfect di-hexahedral habit. The feldspars are 



