ASSOCIATED ROCKS OF PORTHALLA COVE, 467 
examined from a boat. There are several small coves or “ zawns,” 
excavated by the waves in the steeply inclined beds of serpentine for 
the most part, the projecting points and headlands being composed 
of the harder hornblende schist. In all there are, I think, five 
distinct bands of serpentine visible between the quarry and Dranna 
Point. 
Analyses of the Serpentine.—The following Table shows the chief 
variations in composition of the serpentines of Porthalla. I add for 
comparison Mr. Hudleston’s analysis of the black serpentine from 
Cadgwith (Q.J.G.S8. vol. xxxiii. p. 925), from which it will be seen 
that there is no essential difference from a chemical point of view, 
whatever may be said as to their diversity of origin, and much as 
they differ microscopically. 
aa a. b. Gr d. é. 
acendost at LOOCC. 06.6. .<0 ae | 0°46 055 1°85 ‘ 
»  Onignition......... | AEE eh) SHO Lee 
ie eederccmible ....... l 38:60 87-15 30:50 4335{ 98) 
PAM AMIE Gye o/h es Soe ois wie sas cee 0:10 560 508 4:08 1:02 
EEHOUSIOXIMS: .5-.c.eccae. ese . F 8:80 : 4:66 
meeps ee, | 1155 | Lio} athe Lb 74 331 
LUNE. -seabe Jee cea eee trace 0-10 trace 1:51 ied) 
WERT STC) ae ee 33°62 32°30 §=684:65 2843 36-40 
HPC RMCIOE AEG O\s22A:0c8teelecees ) | Seaeted, 9 eddie jaiaees weds 0-41 
Mastek MT OM 20 3.6 vic sjasicine'ajstl), Snedeeleyi ul WYN Nes aake ey tae OM 0-59 
Alkalies and loss ............... J31 0:29 O10, pe AiS i ea eee on 
100:00 100°00 100-00 100:00 100:58 
Sp. gravity.....cccccccseeeeee. 2:65 256 2545 2-644 
There is nothing very remarkable about these results considered 
as analyses of rock-masses of serpentine. The first 1s somewhat 
more than usually charged with magnetic oxide of iron, of which 
many crystals are visible under the lens; while the second, third, 
and fourth contain unusually large amounts of alumina, traces, as I 
believe, of the original composition of the rock whose alteration has 
resulted in the production of the serpentine. The granular structure, 
I believe, tells the same tale. 
Analyses of the Hornblende Schist—Of course this rock varies 
greatly in composition according to the predominance of one or 
other of its chief constituents. Analyses are here given of a speci- 
men of medium appearance (a), to which I have added for com- 
parison analyses of the hornblende schist of the “ Lizard” and of 
Pradanack, by myself and Mr. Hudleston respectively (6, ¢). 
Under d I give an analysis of the hornblendic constituent of the 
Porthalla rock, and under ¢ of the felspathic constituent, each having 
been separated as carefully as possible before analysis. In f I have 
shown what would be the composition of a rock consisting of equal 
ld is greyish-green and granular; 6 dark oil-green; ¢ reddish-brown and 
granular; @ dull dark red, all from Porthalla; e is from Cadgwith. 
