O PROF. T. G. BONNET ON THE HORNBLENDIC AND OTHER 



northern side they are hornblende schist. The upper part of this is 

 rather homogeneous in structure and very dark in colour, the bedding 

 (with the foliation) dipping at about 20° to W. or a little S. of that. 

 There are occasional " eyes " of a white f elspathic mineral ; and this 

 structure becomes more marked as we proceed (descending in the 

 series) along the coast, becoming very'marked at alittleheadland where 

 the dip is rather to the N". of W., and more gentle. There is evidently 

 a roll over in the beds near Penera Head, as noted by Sir H. De la 

 Beche. We were not able to examine this part of the coast very 

 minutely ; but the general character of the rocks agrees with that of 

 the middle or hornblendic group of the south. The first mass of 

 serpentine is in a small cove between Maentallach rock and Polkerris 

 Point ; but I reserve for the present the description of this and of the 

 larger mass to the north *. We traversed Polkerris Point rapidly, 

 but all the rock which we saw appeared to be generally similar to that 

 first mentioned, and we scrutinized carefully the interesting coast- 

 section between this and the village of Porthalla. It will be more 

 convenient to describe this in the reverse direction. The cliffs on the 

 northern side of this cove consist of a brownish shaly or slaty mud- 

 stone, at times slightly micaceous. This may be traced on the 

 southern side of the cove for a few dozen yards, and then the rocks 

 are completely smashed up by a great dislocation, on the other side 

 of which we find the metamorphic series. The lowest rocks seen con- 

 sist of compact chloritic or micaceous schists of a greenish colour, 

 which much resemble the lowest group of the south coast; but 

 among them is a marked granitoid band a few feet thick, exactly 

 like one of those which further south characterize the uppermost 

 group. It dips very sharply away from the fault (?. e. to the east or 

 south-east) ; but the disturbances due to this and to the intrusive mass 

 of serpentine just beyond make it impossible to place any confidence 

 in observations. The ground also is a good deal masked by fallen 

 debris, grass, and heather ; but after much scrambling up and down 

 the hillside on the western edge of the serpentine, I came to the 

 conclusion that these micaceous beds underlie normal hornblendic 

 schist, which is well exposed in the upper part of the hill to the 

 west of the serpentine. It is much roiled and contorted, with a 

 strike roughly varying from the W.N.W. to the N.N.E. 



The serpentine is well exposed, both in a small quarry on the 



an aggregated mass of minute prisms and belonites, and is evidently a secondary 

 product. The more distinctive forms are undoubtedly hornblende, and the 

 general outline of the patches suggests that it has replaced diallage. There are 

 some iron-peroxides, among which ilmenite can be recognized. The slide is 

 rather rich in apatite, the longest prisms being - 08". The general aspect of this 

 rock corresponds fairly well with that of the district near Coverack ; but when 

 I examined the latter I was under the impression that the gabbro was cut by the 

 greenstone (p. 913). I was, however, obliged to traverse the ground rapidly, 

 and did not return, as the relations of these rocks had no bearing on the main 

 objects of my paper, so I may have fallen into error. 



* There is also a little gabbro of the ordinary type, but I did not see a good 

 exposure. 



