324 



EEV. E. HILL ON THE EOCKS OF 



2. The Hornblende-schists. — The hornblende-schists, which form 

 the greatest part of the island, may be studied in any of the acces- 

 sible bays, but most conveniently at Port dn Moulin, on the north- 

 west coast, where they are also most typically displayed. Here 

 cliffs about 150 feet in height, with an aperture pierced near their 

 summit and a quarry at a somewhat higher level, give full oppor- 

 tunities of examination. The rock is beautifully banded in alter- 

 nate dark and pale layers, which are often so fine and thin that an 

 inch will contain four or five. The pale stripes are the narrower, 

 and consist of felspathic material, with some occasional quartz. The 

 darker bands consist, to the extent of two thirds at least and some- 

 times almost entirely, of hornblende in fine irregular grains, perhaps 

 about '02 inch in longer diameter. At the base of the cliff the 

 hornblendic bands thicken up to a foot or so, and have less admix- 

 ture of other constituents. Their grains are coarser, reaching 

 g inch ; and a parallelism in their lie gives a silky lustre on surfaces 

 when split. In some of the lowest beds occur patches of aggregated 

 lustrous black hornblende, in pieces J inch long • as these are sur- 

 rounded by a whitish border they are probably segregations. The 

 general parallelism of the layers is not everywhere complete ; some 

 laminae are of varying breadth, others even lenticular ; here and there 

 a layer inclines down towards, and unites with, that below ; thus 

 there are indications of current-bedding, and these are confirmed by 

 a section in the cliff at the northern corner of the bay, so clear and 

 large as to leave no room for doubt (fig. 2). 



Pig. 2. — " False-bedding " finely shoivn in Hornblende- schists at the 

 north-east corner of Port du Moulin, Sark. (Height about 

 12 feet.) 



The beds here, at Port du Moulin, lie almost horizontal; the 

 very gentle dip they have is westerly. 



The beds of nearly pure hornblende just described lie in perfect 

 conformity and continuity upon a thick bed of somewhat different 

 appearance. This is composed, to nearly half its amount, of felspar 

 in grains of about | inch across, hornblende being only the second 

 constituent in amount ; black mica also occurs plentifully in rather 

 minute flakes. At Port du Moulin a thickness of about 10 or 15 feet 

 of this bed is exposed at low tide, but in a bay J mile further 

 south there is as much as 30 or 40 feet ; no lower rock can be seen at 



