Vol. 52.] "ROCKS OF THE LIZAKD DISTEICT. 47 



change from the normal to the abnormal type. Still, the mass as a 

 whole exhibits a little more variety in composition than is usual in 

 the hornblende-schists. Gneissoid rocks occur in it, as near Old 

 Lizard Head and on the south side of Holseer Cove, and masses of 

 a brownish mica-schist at Polledan Cove (discovered by Mr. Pox), 

 Polpeor Cove, and in the headland south of Holseer Cove. This 

 last I bad not before observed. It is seen, under the microscope, 

 to consist of quartz and mica, mostly biotite, which, as in the 

 similar rock in Polpeor Cove, has evidently suffered from pressure, 

 and it contains grains, not very regular in shape, of a water-clear 

 mineral, evidently of secondary origin. This may be felspar, but 

 the refraction-index seems a little high for that mineral, and one or 

 two grains show a slight pleochroism (reddish), so I think it more 

 probably andalusite. 



On the northern side also of Holseer Cove are two rather peculiar 

 rocks. One is a slaty, purplish-coloured rock, which is found, on 

 microscopic examination, to consist of grains of a clear mineral, 

 much of which is isotropic (? opal) with a greenish mineral, 

 extinguishing obliquely (probably a chlorite), ferrite, and opacite. 

 The mode of occurrence in the field suggested the possibility of an 

 intrusive origin, but I cannot say that the microscopic examination 

 throws any light on this question. Nor does it help us with a 

 purplish, slightly-streaked rock at the head of the Cove, which still 

 more resembles an intrusive mass. It consists of grains of the 

 above-named clear mineral, flakes of biotite, more or less altered, a 

 little white mica, grains of iron oxide, and a few very small garnets. 

 Both these rocks, whatever may have been their origin, are now 

 schists, and with that group we must leave them. 



(b) Age of the Lizard Rocks. 



Though I now think it more probable that the hornblendic as 

 well as the granulitic groups are not altered sedimentaries, but 

 peculiar forms of igneous rocks, I am more than ever convinced 

 that they are older, perhaps very much older, than the serpentine 

 and the other rocks intrusive in it — that is, I still consider them 

 to be ArchaBan. It is needless to recapitulate arguments which 

 have been more than once stated in print, but I may say that 

 the resemblance of some of the gneissoid rocks of Sark and Britanny 

 to those at the Lizard, and the practical identity of the hornblende- 

 schists in the first and last localities, make it highly probable 

 that they do not differ materially in age. Now, the gneissoid rocks 

 of Britanny are older than the Cambrian, older even than the 

 c Schistes de St. L6 ' ; while it is almost certain that the hornblende- 



remainder. This, I may add, is not the only instance in which a like incorrect 

 appeal has been made to the evidence of ' dynamo-metamorphism ' at the- 

 northern and the southern boundary of the district. — January, 1896.] 



