138 EEV. E. HILL AND PEOF. T. G. BONN^EY ON THE 



hornblende-scliists) of Sark, however closely they may simulate 

 stratification. 



As an hypothesis to explain the whole series — hut this, it must he 

 remembered, is advanced only as an hypothesis — we suggest that 

 formerly two magmas existed, the one now represented by the aplite 

 or granite of the veins and the bands similar in composition, the 

 other by the lumps of coarse hornblende ; that after the latter 

 had become solidified, at least locally, the more acid rock came in, 

 brecciating it here, more or less melting it there, the two flowing on 

 together, at any rate in places, and producing the structures which 

 have been described in this paper. In some cases, as in the more 

 finely banded gneisses and the more uniform masses of hornblende- 

 schist, the one magma or the other dominates, though in it a small 

 portion of the other may have been dissolved, but in others it by no 

 means follows that either the one rock or the other was wholly liquid. 

 From what has been said, it is obvious that very probably the crystals 

 in the lumps of coarse hornblende-rock had been formed anterior to 

 this period, but other parts of the same rock may have been partially 

 melted. The more granitic rock also may have contained its own 

 crystalline constituents at the time of its intrusion, so that in the 

 mass as a whole almost every stage may have existed from a viscous 

 fluid to a holocrystalline solid.^ 



By what interval of time the phases, described above, were 

 separated, there is nothing to show. It is not, however, very 

 probable that a distinctly acid rock, such as the aplite, would follow 

 immediately after one no less distinctly basic such as the ' horn- 

 blendite ' or certain parts of the hornblende-schist. jN'or is it 

 possible to say at what depth beneath the surface these fluxional 

 movements occurred. Some of the phenomena are suggestive of a 

 comparatively unimpeded flowing over a considerable area, but the 

 crystalline condition of the rocks indicates a rather slow cooling, so 

 that we must assume either exceptional conditions, if the depth below 

 the surface was great, or a much more rapid increase of crust 

 temperature than now prevails generally, if it was but small. There 

 is nothing to connect the movements with any known epoch of 

 mountain-making ; they were long anterior to the great post-Car- 

 boniferous disturbances.^ 



^ To prevent misunderstandiBg, it may be well to state that the Authors 

 (after another visit) adhere to their opinion as to the origin of the foliation in 

 the great mass of gneiss forming the southern part of Guernsey and are fully 

 convinced that this is a pressure-structure> the original rock having been a por- 

 phyritic granite. It may be of interest to mention that in a dyke of diabase 

 vphich cuts the ' wriggling ' banded gneiss on the western shore, rather to the 

 south of the actual ridge of the Coupee, they found 2 or 3 fragments of an augen 

 gneiss, which reminded them of the rock of Lihou or Leree Eay in Gruernsey. 

 This is the only case where they have seen anything in Sark to recall these, the 

 oldest rocks, as they fully believe, in the Channel Islands. 



2 Hill, Quart. Journ. Geol. Soc. vol. xlv. (1889) pp. 387-89: Bonney, iUd, 

 vol. xhii. (1887) p. 319. 



