SCHISTS OF THE LIZARD DISTRICT. 21 



But if the Pebidians of St. Davids are to be taken as typical of that 

 group, and if we are to put any trust at all in conclusions derived 

 from lithological studies, we must assign a greater antiquity than 

 this to these Anglesey rocks. Hence, as I consider that we are not 

 yet in a position to offer a definite classification of the members of 

 the Archaean series, I will say no more than that I consider these 

 Lizard rocks to belong to some part, and that by no means the latest, 

 of the record of that immense period of time. 



Y. Further Notes on the Serpentine. 



I was obliged to leave two outlying patches of serpentine unde- 

 scribed in my former paper on the Lizard district, but am now able 

 to give some particulars concerning them. They are indicated on the 

 Geological map, the smaller patch at Polkerris, the larger close to 

 Porthalla. The former of these is best examined on the hill on the 

 northern side of the cove, it being imperfectly, if at all, exposed on 

 the shore. It is a compact variety, with a curd-like fracture, its 

 colour varying from dull reddish brown to dark green, and is tra- 

 versed by numerous joints (so that large specimens cannot be 

 obtained) which are coated with steatitic films of green and red 

 colour. A microscopic section exhibits the usual network structure 

 of doubly refracting " strings," with an isotropic interior, which has 

 been fully described in my former paper. We find the usual 

 clustered granules of opacite, ferrite, &c, and one crystal, now 

 replaced by serpentinous minerals, which I have no doubt has been 

 bronzite. 



The serpentine near Porthalla can be examined on the hill- 

 side south of the cove and at the foot of the cliffs (at low tide). 

 The junction with the hornblende schist was excellently shown at 

 the time of my visit in a small quarry on the latter and in several 

 places on the former. Though occasionally the compact nature of 

 the schist, and the changes produced by weathering or by the secon- 

 dary deposits of films of steatitic minerals, render the actual junction 

 slightly obscure to the unaided eye, it can be detected in the field 

 on closer examination, helped by the knife and hammer, and is in- 

 dubitable when the microscope is used. 



This serpentine varies from a dull purplish green to a greenish 

 grey, the latter colour denoting the more weathered specimens. 

 Sometimes it is a homogeneous dark rock, at other times it exhibits 

 a very marked streaky aspect, reminding one of the fluidal structure 

 of rhyolites. I have examined microscopically four varieties : — 

 (1) from the compact dark serpentine, (2) from the grey-green one, 

 (3) from a markedly banded specimen, (4) from a similar rock with 

 small " eyes " of a filmy talcose mineral. The general character of 

 the ground-mass resembles that of the last- described and of the 

 more compact serpentines further south, the differences being only 

 varietal. I consider the rock to be an altered peridotite, but to 

 have been originally rather finely crystalline. No crystals of 

 enstatite, hornblende, or augite are now recognizable; but some 



