48 PEOr. T. G. BONNET AND THE EEY. E. HILL : [Feb. I912, 



On that occasion Prof. Bonney wrote in his diary that, on the whole, 

 the rock reminded him more of a dark felsite, like some in Wales^ 

 than of a diabase. jN'othing in its aspect suggested a sedimentary 

 origin, but he observed, when collecting a specimen for microscopic 

 examination, that it was exceptionally hard, and had a more irregular 

 fracture than is usual wdth igneous rocks of similar texture. The 

 first glance at a slice showed that we had been deceived, and we 

 afterwards found that the late Father ISToury had recognized the 

 clastic origin of the rock, for in a sketch-map of Guernsey, con- 

 tributed to the third edition of Ansted's ' Channel Islands,' ^ he 

 marks it as schiste. We subsequently came upon a remark in 

 his ' Geologie de Jersey' 1886, p. 127, which had escaped our 

 notice : — 



' La pointe ouest de Gruernesej coutient cependant, au sud du Forfc Pezeries, 

 un petit massif de schiste semblable a celui de Jersey et durci par les granites 

 qui forment cette extremite de I'ile.' 



But these granites can only be the gneiss, which we have no 

 doubt is the older of the two, and, notwithstanding the lax use of 

 the term schiste, we should say that, as a rule, it was hardly 

 applicable to the Pleinmout rock. 



!Mr. Hill paid two short visits this year (1911) for further exami- 

 nation. The grit forms cliffs and slopes for about half a mile, from 

 the small bay west of Fort Pezeries to somewhere near the spot 

 marked Pleiomont Point. In the bay there is, on the east, gneiss ; 

 on the west, grit : at high-water mark these are separated by 

 shingle ; they come together in the rocks bared at low tide, but 

 are much masked by seaweed. Some hundred or so yards from the 

 shore a dark crystalline dyke^ is seen, crossing both gneiss and 

 grit ; also, a pink dyke (described below, B, p. 50), some 12 feet 

 broad, cutting all three. On a small knoll in the shingle he came 

 upon a junction of a rock, rather resembling a gneiss, with grit; 

 but the former proves, as will be seen, to be a variety of the ' red 

 dykes ' (described below, C, p. 50). 



On land the boundary between gneiss and grit can be followed 

 southward by outcrops. From a high level it descends somewhat 

 suddenly to the sea, and at a spot which can be reached a contact 

 is seen. The grit here for a few inches contains small fragments, 

 and shows a slight banding (see below, p. 49). The gneiss at this 

 spot lies on the grit : not far south all the rocks are gneiss ; and 

 opposite the contact-spot, only 20 yards seaward (west)> there is 

 again gneiss. A cleft, no doubt a fault, forms the seaw^ard boundary 

 between the two rocks here, and as far as it was followed north. 

 There has apparently been great disturbance. 



1 Eevised by E. T. Nicolle (1893), the first edition being in 1862. 



- A slice shows the rock to be mainly composed of a small prismatic green 

 hornblende, rather acicular in structure, with colourless needles, apparently 

 the same mineral, piercing a water-clear ground-mass, probably a secondary 

 felspar. Here and there is a little epidote, sometimes rather impure. As the 

 structure is very likely a result of pressure, the rock may be named an 

 epidiorite. 



