10 



PROE. T. G. BONNEY ON THE HORNBLEND1C AND OTHER 



diameter. Sir H. De la Beche mentions the occasional occurrence 

 of fragments of hornblende schist; but as he sometimes permits 

 himself (as was at that time inevitable) a little laxity in the use of 

 the term, I was anxious to settle a point obviously of the first 

 importance with relation to the age of the two series. After a 

 prolonged search, I at last succeeded in discovering an indubitable 

 fragment of hornblende schist, and so corroborating his statement. 

 The fragments are rare ; for it was not till my second visit, and 

 after searching for at least an hour, that I found it. The quartz- 

 felspar pebbles also clear] y come from a metamorphic series, if not 

 from the exact horizon that I have mentioned, so that there can be 

 henceforth no question that the two series are separated by an 

 enormous interval of time, and that there is no approach to a 

 passage of the slaty mudstones into the hornblende schist. 



Section in Polurrian Cove, 

 a b c 



a. Slate. 



b. Breccia. 



c. Schist, /. Fault. 



Turning now to the junction of the slaty with the hornblendic 

 series in Polurrian Cove, on the western coast, we find similar 

 evidence of the complete separation of the two. The former 

 is a smooth, satiny, blackish or brownish slaty rock, veined 

 occasionally with quartz and calcite, in which are lenticular 

 sandy bands, dipping generally about 30° E.S.E. The horn- 

 blendic series is very massive, resembling at first sight a decom- 

 posed greenstone, but disclosing on closer examination indubitable 

 signs of foliation and bedding: it dips at about 70° S. At the 

 junction is a fault-breccia, a yard or two thick, composed mainly, 

 if not wholly, of fragments of the newer series in a rusty- 

 looking matrix*. The above diagram shows the general rela- 

 tions of the two series. The two rocks on opposite sides of the 

 fault are as different lithologically as is possible, the one being 

 almost unalteredf, the other as highly metamorphosed as is 

 possible. . 



* This is described by Sir H. De la Beche, though with much hesitation, as 

 a conglomerate. I think that there is a. better exposure now than on the occa- 

 sion of his visits, and that there can be no doubt on the matter. 



1" I believe there is a true cleavage, but correspondent with the bedding ; the 

 surfaces of this have been rumpled by subsequent lateral pressure, which has 

 left its mark on the microscopic structure of the rock. This would be termed a 

 " phyllite " by many continental geologists, and much resembles some specimens 



