210 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of the variety above described, the (h'stribution of the dark mineral 

 through the groundmass from one cause or another, renders the hitter 

 quite dense in appearance and of a peculiar grey or bluish-grey color. 

 The phenocrysts are more conspicuous and all the more so when the 

 feldspar is whitened or slightly stained (pink or brown). This type 

 of rock is abundantly exposed for observation on the broad, gently 

 rounded, smooth ledges characteristic of the northerly slopes of the 

 hills and is a very characteristic feature of the area. 



This last rock is scarcely to be distinguished from the phase with a 

 truly dense groundmass into which it grades, and which also has a wide 

 but irregular areal distribution. If anything the phenocrysts are 

 slightly less abundant than in the former, the quartz forms rounder 

 and more prominent grains and the groundmass is more varied in 

 color and often darker. The color may be dark-grey, l)luish-grey, 

 pale-green or even dark-red to brown and less commonly of a purplish- 

 red. The last mentioned colors belonging to exposures which have 

 suffered a more complete oxidation of the iron-bearing minerals. 

 Many ledges exhibit a heterogeneous structiu-e. The grey or bluish- 

 grey porphyry contains angular fragments, usually of the yellowish- 

 green color, varying in size from ones measuring a few millimeters 

 across to ones upwards of several inches in their largest dimension; 

 also numerous streaks and blotches of the similar material, of varying 

 width and length. These streaks and fragments by their arrangement 

 often show a pronounced flow structure; again the fragments are so 

 numerous as to constitute a l.)reccia. Alteration further accentuates 

 the heterogeneity. At one point on Heminway Hill the two varieties 

 are so mingled together as to produce the appearance of a tuff. The 

 brecciated character of the porphyry is clearly connected with the 

 immediate contact of the magma with older rocks and is sufficient 

 evidence that the contacts were at most only a short distance from 

 the present surfaces of the rocks as now exposed, and that erosion 

 has removed very little of the intrusive rock, a point that seems to be 

 entirely borne out by the notable scarcity of rocks of the alkaline 

 type in the conglomerates and other sediments of later age. The 

 streaked porphyry referred to by Professor Crosby ^^ as occurring 

 on the south side of the Blue Hills both in situ and in the basal beds 

 of the Norfolk-Basin conglomerate is undoubtedly of this type and 

 its heterogenous structure is not, as was supposed by him, due 

 entirely to differential weathering. 



32 op. cit. p. 359. 



