32 



PROF. T. G. BONNEY ON" THE 



[Feb. 1896, 



of expressing my admiration of the map published in Messrs. Fox 

 and Teall's paper. 1 The making of it must have been no easy task, 

 for the slope is very irregular, broken, and sometimes not the place 

 for an unpractised climber. So accurate are both it and their descrip- 

 tion that I had no difficulty in following them step by step in going 

 over the ground. This, then, is how I interpret the sections. 



Nowhere in the serpentine can I find any of those symptoms, so 

 familiar to me, which indicate that after solidification it has 

 been subjected to severe mechanical disturbances (dynamo-meta- 

 morphism. The two cases, figured by my friends 2 (and the number of 

 such is small), appeared to me due to a different cause. The intru- 

 sion of a hot viscid mass into one already solid suffices sometimes 

 to bend either the latter locally or pieces of it which have been torn 

 off. 3 Here there is no sign of the two rocks being folded as solids. 



Furthermore, here and there, the serpentine includes slabs of 

 hornblende-schist of normal aspect, or the junction of the two rocks 

 may be often seen (as will be presently described), to be perfectly 

 welded and yet distinct. In one case (fig. 9) the serpentine runs up to 

 the broken edges of a wedge- 

 like projection of the horn- Y ig. 9.— Banded hornblende-schist 

 blende-schist. In the latter in serpentine, Potstone Point. 



rock the banding is regular, 

 only a little rumpling and 

 flexure being perceptible at 

 two places, which are respec- 

 tively 2 feet and 4 feet in a 

 line from the point of the 

 former rock, while the mass as 

 a whole is not affected. The 

 disturbance, in short, reminds 

 one of the crumpling which 

 might be produced by thrusting 

 the thicker end of a flat paper- 

 Imife between the pages of a 

 tightly-closed book. The ser- 

 pentine in parts of this < com- 

 plex ' appears to be sometimes 

 impure and harder than it 

 ought to be ; often it is streaked like a brownish slag, while here and 

 there a thin band can be discovered, which resembles hornblende- 



The dotted rock represents serpentine, 

 the lines indicate diagrammatically 

 the banding of the hornblende- 

 schist. 



1 Quart. Journ. Geol. Soc. vol. xlix. (1893) p. 203 (I had it with me as I 

 worked). Once or twice I felt doubtful about the small faults, and should have 

 made a slight change as to a dyke ; but these are insignificant details, and I, 

 not they, may be wrong. 2 Op. cit p. 212. 



3 Which explains figs. 1 and 2, op. cit. p. 202. Under these circumstances the 

 junction-surface of the two rocks might be ' wavy/ and ' for every tongue of 

 serpentine into schist [there would be] .... a corresponding tongue of schist 

 into serpentine.' In such cases also the two rocks would probably exhibit a 

 parallel foliation. This does not appear to have occurred to my friends, and I 

 find no evidence that they minutely investigated the structure of the serpentine, 

 in which, however, the key of the position is to be found. 



