126 EEV. E. HILL AND PEOF. T. G. BONNEZ ON THE 



grained type is more common, though the coarser also occurs, e. g. 

 to the N. of Saignie Bay, and the former, so far as we have noticed, 

 seems to underlie the hornblende- schist ; apparently, however, its 

 thickness is variable. The most characteristic examples are to be 

 seen about Port du Moulin and in Saignie Bay. But, as already said, 

 we do not consider any very precise subdivision to be possible ; 

 nevertheless a recognition of the distinction may be of some use 

 for general purposes. 



The coarser rock consists of layers of rather dark brownish and 

 pinkish-grey colour. In some localities, as on the north-eastern 

 coast, on the west side of the Coupee and at Port es Sales, these 

 layers are wonderfully ' gnarled ' ; — in others {e. g. about Dixcart 

 Bay) the rock appears to have been somewhat crushed after the 

 characteristic structure had been assumed. As an example we may 

 select a beautifully wrinkled specimen from the southern side of 

 Port es Sales. In this the dark layers (as is not infrequent) slightly 

 predominate over the light. The former occasionally are about 

 I inch in thickness, the latter generally rather less, but of course 

 the ' wrinkling ' has produced local thickening and' thinning. The 

 structure is beautifully exhibited in a slice rather more than 2"x 1" 

 prepared for microscopic examination. It is obvious, at a glance, 

 that the contortions have been produced after the rock, whatever 

 may have been its origin, had become banded and crystalline. By 

 following up the micaceous bands we can see that the mica-flakes, 

 which once lay roughly in the direction of the bands, have been 

 twisted up and disturbed, till occasionally a ' strain-slip ' cleavage 

 all but sets in.^ 



The chief constituents of this rock are quartz, felspar (orthoclase 

 and plagioclase ^), and biotite, with some iron oxide and a fair 

 amount of apatite. The paler layers consist mainly of quartz and 

 felspar, the darker of felspar and mica. The felspars are granular 

 in form, and occasionally enclose small rounded grains of quartz. 

 The latter mineral has a tendency to occur in rather elongated 

 patches ; in tact one may infer that the structure of the rock was 

 originally very similar to that described above, except perhaps that 

 the felspar grains may have been a little less irregular in outline. 

 But that the rock has undergone some mechanical disturbance is 

 evident, not only, as already mentioned, from the arrangement of 

 the mica-flakes, but also because the felspar grains are not seldom 

 parted by a crack, which has been subsequently filled up; the 

 patches of quartz occasionally exhibit a tendency to streak out, and 

 this mineral here and there occurs in aggregated granules, which is 

 a sure indication that there has been a certain amount of fracture, 

 followed by cementation and reconsolidation. 



^ Obviously there is nothing unusual in this. In the course of my work 

 elsewhere I have come across every stage, from undisturbed banding to the 

 setting up of a cleavage- {'olialion under strain, in gnarled gneisses, as described 

 in my Presidential Address, Quart. Journ. Geol. Soc. vol. xlii. (1886) Proc. 

 pp. ()8-72.— T. G. B. 



^ Microcline does not occur in the slides examined. 



