310 MR. J. PARKINSON ON THE ROCKS OF [May I9OO, 



which grains or partly idiomorphic felspars lie scattered. These 

 grains frequently polarize as distinct individuals. The extinctions 

 of five or six crystals which are cut approximately perpendicular 

 to 010 indicate albite or oligoclase. Neither twinning nor extinction 

 is very regular. The investing felspar is clear, save for faint flecks 

 of brown kaolin, and in many cases it is not easily distinguished 

 with certainty from quartz. It appears to be usually orthoelase, 

 but occasionally a resemblance to the cross-hatched structure of 

 microcline is found. Sometimes quartz takes the place of the 

 embedding felspar ; it never occurs in definite grains, and on the 

 whole is not common. Occasionally a crystal shows itself between 

 crossed nicols to be built up of closely-packed polygonal grains, 

 each polarizing at a slightly different angle from that of its 

 neighbour, and in this case the clear replacing felspar is almost 

 entirely absent. 



The two remaining principal constituents are green hornblende 

 and a mica, usually rather altered, brownish or green in colour. 

 The mica is of late consolidation, and is characteristically embedded 

 between the grains of felspar in irregular flakes. That its for- 

 mation is subsequent to the breaking up of the felspar-crystals, as 

 at Sorel Point, is thus apparent. A very strong resemblance exists 

 between the changes which have taken place in the felspars of the 

 rocks from that locality 1 and those just described. 



An examination of the earlier stages of this process of reconsti- 

 tution shows that solution has been effected with greatest facility 

 parallel to the direction of the albite- twinning, but the blunt 

 rectangles or even squares formed by the replacing felspar indicate 

 that absorption has proceeded almost uniformly in every direction. 



(2) Nature of the Earlier Intrusive Magma. 



It is rather difficult to determine what the nature of the invading 

 magma really was, but there can be no doubt that at the time of 

 intrusion the constituents requisite for hornblende and mica were 

 poorly represented or entirely absent, and that the silica-percentage 

 was not high. 



Sections cut from specimens which show little or no material 

 derived from the older basic 2 rock through which the intruder 

 passed present a structure sometimes like that just described, of 

 which the corrosion of the felspars is the most striking feature. 

 In others such corrosion is absent, and the slide consists of inter- 

 locking and mutually interfering felspars, with no quartz, a little 

 hornblende, and presenting a gneissose appearance. These felspars 

 are so opaque that it is difficult to determine the species, but they are 

 for the most part a plagioclase. The hornblende, in the majority 

 of cases, is probably derived by a process of absorption from the 

 diabase. We may seek an explanation of these structures in the 



1 Quart. Journ. Geol. Soc. vol. lv (1899) p. 437, figs. 1 & 2. 



2 That is, no augite and little or no hornblende or mica (derived as a 

 product of hornblende with felspar). 



