REPORTS AND PROCEEDINGS. 
Royat GEOLOGICAL SOCIETY OF CORNWALL. 
November 14, 1920. 
“On the Geology of St. Michael’s Mount.” By E. H. Davison, 
B.Sc., F.G.S. 
The granite of the island is not of normal character, but shows 
idiomorphic crystals of quartz afd felspar in a fine-grained crystalline 
ground-mass. The predominant type of alteration is greisening, the 
granite being traversed by a series of narrow parallel veins of greisen, 
with a central portion of pegmatite or of quartz, which carries such 
minerals as cassiterite, blende, wolfram, chalcopyrite, molybdenite, 
apatite, lithia mica, fluor, tourmaline, beryl, etc. The slate in 
contact with the granite shows evidence of intense thermal meta- 
morphism, and in some cases there is evidence of the granite 
having absorbed the slate along the contact. Impregnation of 
the slate by minute granitic veins is also common. The lodes are 
seen in the form of greisen veins carrying mineral and in the form 
of more typical fissure lodes, and they occur in great numbers in both 
granite and clay-slate, but are all narrow. A clean ferruginous sand 
occurs in the island, which in many respects resembles St. Erth 
Sand. Tourmalinization is the exception, and the author concludes 
that the veins were filled by magmatic solutions which were highly 
siliceous and contained fluorine to a considerable extent, but little 
boron. 
“ Note on an Alluvial Pebble from Nanpuska.”’ By HE. H. Davison, 
B.Sc., F.G.S. 
The special interest of the pebble lies in the relationship of the 
quartz and cassiterite. The pebble is triangular, the apex being 
_ formed of a small mass of dark-brown zonal cassiterite, with quartz 
above, through which needles of cassiterite are arranged radially. 
The microscope shows the quartz to be a mosaic, and the quartz and 
cassiterite needles must either have crystallized simultaneously or 
the needles crystallized in a solution of silica sufficiently viscous to 
hold them in position. 
‘* Note on an occurrence of Granite at Bridge.’ By H. H. Davison, 
B.Sc., F.G.S. 
In a quarry at Bridge, 12 miles north of the margin of the Carn 
Menelez granite, a quartz porphyry dyke was seen to contain a large 
xenolith of granite just above the foot wall. The granite showed no 
signs of having been melted along its margin, and seems to have been 
carried but a short distance. This confirms the view that the slope 
of the Carn Menelez granite mass to the north is very gentle. 
