476 PROF. T. G. BONNET AND MAJOR-GEN. C. A. M'^MAHON 



A structure which would certainly leave its mark, and probably 

 produce a serpentine, such as some of those mentioned above, is 

 described and figured by Prof. Eenard in his description of the 

 peridotite of St. Paul's Island *. The rock is rather fine-grained 

 and contains " eyes " of enstatite. The author, by an admirable 

 piece of inductive reasoning, proves that the rock exhibits a fluxion 

 structure and is of igneous origin, but abstains from accepting the 

 conclusion because it has been asserted by certain authorities that 

 some peridotites are metamorphosed stratified rocks. 



III. The Rocks older than the Serpentine. 



These were divided by Prof. Bonney, in 1882, into three groups, 

 between which, however, no sharp line of demarcation was 

 supposed to exist. Of these he considered the Granulitic to be the 

 upper, the Hornblendic the middle, and the Micaceous the lower. 

 He thought the series, as a whole, had been sedimentary in origin, 

 but that the hornblendic rocks were probably altered tufiB, and, in 

 some cases, might even have been basic lavas. In 1888, Mr. Foxl* 

 communicated to the Society the results of his examination of the 

 islands fringing the south coast of the Lizard, together with petro- 

 graphic notes by Mr. Teall; this was followed in 1889 J by a 

 paper from General M'^Mahon on the granulitic and hornblendic 

 rocks. It will, therefore, suffice on the present occasion to refer 

 for details to these and other papers, and indicate in general terms 

 the problems which are presented for solution. 



(1) The Granulitic Growp. 



This group, as has been said, is characterized by a dark dioritic 

 rock, veined or interbanded by a lighter one, which resembles a 

 rather fine-grained granite. The former is not seldom porphyritic, 

 the structure setting in and disappearing in an irregular way, so 

 that a mass is " spattered " with felspar crystals, as is the face of a 

 rock by the marks of a charge of shot. A slight foliation can often 

 be discerned in the groundmass. Porphyritic felspar, as we now 

 know, makes an igneous origin at least probable ; but the matrix 

 (which consists mainly of rather rounded or slightly elongated 

 grains of felspar and hornblende, with more or less biotite§), so far 

 as we are aware, has not disclosed any characteristic structure. The 

 lighter rock is very closely allied to a vein-granite. The felspars 

 are not idiomorphic, but occur, like the quartz, in rather rounded or 

 elongated grains. The history of the rock, so far as regards its 

 macroscopic character, can be best studied in Kennack and in Pen 

 Yoose Coves. It will suffice to recapitulate the principal facts of 

 which any theory as to the origin of the group must take account. 



* * Challenger' Eeports, Narrative, vol. ii. 



t Quart. Journ. Geol. Soc. vol. xlir. (1888) p. 309. 



+ Ihid. vol. xlv. (1889) p. 519. 



§ Apatite, sphene, and magnetite are present in nearly all slices. 



