170 PROF, W. J. SOLLAS ON A PROCESS FOR [May 1902, 
The chemical composition, as calculated from these results, is given 
in column I below; in columns IJ, IIT, and IV are the results of 
analyses of Cornish granites given by the late J. A. Phillips.’ 
ibs IT. III. IV. 
[Penrhyn.| [Carn Brea.| [-Botallack.] [ay oy 
(Caleulated.) (Found.) (Found.) (Found.) 
SOS eects 7401 74°69 74:54 70°65 
PU 0 ere 14-28 16°21 14°86 16:16 
Pe ects ae: 68 trace 2°53 1:53 
LSC) aera ae 1:86 1-16 0:28 0:52 
WTO eo ianets 0-14 0:58 — -- 
MEO! Fee. 0-14 0:48 _— — 
ORO. Freee ee s: 0:27 0:28 0-29 0°55 
TS OD tye xt Cove 4-76 3°64 3°73 8°66 
GAO | ans .o.. Seren 2°32 1:18 3°49 0°54 
LUT O Mae Seer 0:08 0-10 trace _ 
1a Oe nearer oe O71 1:23 0-87 1-22 
beeen nea 0°41 -- aa — 
LER ie ae 0-10 — = 
99°76 99°55 100-54 99:83 
Specific gravity... 2°68 2°64 2°66 2-02, 
The concordance between the chemical composition found directly, 
as given by columns II & III, and that calculated from the 
mineral composition, leads to the inference that the rocks analysed 
by John Arthur Phillips possessed a mineral constitution similar 
to that of the rock examined by me: yet no two of the specimens 
came from the same locality. It would thus appear that the 
Cornish granites possess, and maintain over a wide area, a con- 
siderable degree of uniformity. 
A comparison of the mineral composition of these granites with 
those of Leinster may be made from the following table :— 
| 
naa LEINSTER | LEINSTER Bao 
eens | Turee-Rock) Avcuriu | Tae 
a a GRANITE. | GRANITE. D gine 
IORITE. 
Orthoclase 2 ......... 24°62 15°59 | 9°19 3°80 
PiptGe. . 34 eee ee 13°42 30°16 37°14 30°96 
{ Amartinite” caccee. selon wegen 9°17 14:00 20°64 
COTE ree Hee 40°23 36°16 | 31°89 17:00 
Muscovite ............ 10°09 ye : 
Re ck 11-46 } 808 i 21-00 
Hhornblend@ .....00-:| scaex “4. sctece OUD eeeEEee 4:80 
Magnetite and heavy 
mimerals...........: ORG a Ae 0°55 8 1:70 
1 Quart. Journ. Geol. Soc. vol. xxxi (1875) p. 330. 
2 The potash-felspar is orthoclase in the Cornish granite, microcline in 
that of Leinster. 
= Sphene and epidote in this case. 
