40 



PEOr. T. G. BONNET AND THE RET. E. HILL ; [Feb. I9I2, 



Fig. 1. — Junction-surface of 

 dyke and. gneiss. 



may reach or even exceed a dozen feet in width ; many are not 6 feet 

 wide, and they may thin down to less than an inch and yet 

 be quite distinct. Most of them are microgranitic in structure, 

 consisting of quartz and a red felspar with, as a rule, a minute 

 sparsely-distributed greenish material ; and thus they are prac- 

 tically finely-grained aplites. One or two, however, are a little 

 more distinctly granular, with a ferromagnesian constituent more 

 obviously present.^ 



i^ear the north-western angle is a very compact instance, and 

 one or two dj'kes - a little farther south have chilled edges ; but, as 

 a rule, no change in texture is obvious on approaching a junction. 



That is generally clean, with a close 

 weld, but now and again these 

 dykes have locally melted down 

 both the gneiss and the diorite, 

 and have incorporated some of 

 their material. Of the former rock, 

 the felspars more or less survive, 

 as in the diorite already described, 

 though distinguished with more 

 difficulty owing to similarity of 

 colour ; while the micaceous con- 

 stituent gives rise to a green 

 streaking. One of the appended 

 diagrams represents a junction- 

 surface of dyke and gneiss (fig. J ) ; 

 the other (fig. 2, p. -il) au inclusion 

 of the latter which, in the upper and 

 thinner part, loses its porphyritic 

 character, as if it had been more or 

 less fused in situ. 



As, however, the streaking of 

 the ' red dyke ' extends for a few 

 feet from the junction, that struc- 

 ture may have been mainly ac- 

 quired in another way. The magma seems to produce fusion in 

 the diorite more readily than in the gneiss, though here also clear 

 closely-welded junctions often occur. The two may be seen in 

 different stages of incomplete mixture. Sometimes we find an 

 elongated, shuttle-shaped patch of ' greenstone' ending in a streak. 

 When the piece is a few inches thick, the inner part resembles a 

 rather fine-grained diorite. Both the outer part and the thicker 

 streaks, which, however, generally do not exceed a couple of inches, 

 are minutely granular ; the outsides and the thinner streaks (from 

 1 inch downwards) look quite compact. These alternate for a 

 short distance with fairly-clean red bauds; while the rest, or even 



^ Mr. Hill, in Septembei' 1911, came upon a dyke, which was a marked 

 example of this variety, at low tide on the harbour side of the breakwater. 



- As will be presently shown, further work makes it probable that these 

 are later in date than the others. 



[A=Augen-gneiss ; the augen are 

 not quite so large as usual. 



B = Border, about 2 inches broad, 

 on the whole more like 

 gneiss. 



C = Eed rock, rather streaky. 



D=Edge of a diabase dyke.] 



