512 G. McPherson, jun., and T. Lamb— 
EVIDENCE BEARING ON THE DEPOSITION OF CASSITERITE, 
The structure of an alluvial pebble of cassiterite found in the 
Nanpusker Valley throws some light on the manner in which the 
mineral was deposited. The pebble consists of a roughly 
hemispherical nodule of cassiterite about half an inch in diameter 
with white quartz attached ; through this quartz rays of cassiterite 
needles run radiately from the main mass. Microscopic exmination 
shows that the cassiterite nodule is crystalline dark brown in colour 
and beautifully zoned. The rays consist of two kinds. Nearest to 
the nodule there are bunches of dark brown cassiterite needles, and 
further away there 1s. a zone of yellow prisms of cassiterite with good 
crystal outlines, while outside these the quartz is crowded with small 
needles and skeleton crystals. The quartz is seen to be a mosaic 
of small irregularly shaped grains, which are in optical continuity 
around the ends of the cassiterite crystals and needles. The structure 
shows that the cassiterite either crystallized contemporaneously 
with the quartz or that crystallization took place in a silica jelly 
sufficiently viscous to hold the needles in position. 
In conclusion, I must express my thanks to Mr. Harvey Kitto 
for collecting information with regard to Killifreth Mine, to ely 
Minerals Company, Limited, for permission to inspect their plans 
and sections, and to the managers of the various mines visited for 
giving me valuable information and every facility for carrying out 
the work. 
Platinum-bearing Rocks in the Lizard District. 
By G. McPrersoy, jun., Stud.I.M.M., and T. Lamp, Stud.I.M.M. 
OME remarks made at a meeting of the Cornish Institute of 
Engineers by Dr. R, H. Rastall directed attention to the 
possibility of platinum being found in the Lizard. In a paper upon 
“Ore Deposits of Igneous Origin ’’, Dr. Rastail pointed out that 
in the Lizard the local characteristics were favourable to the presence 
of platinum, although no definite information of its occurrence had 
been reported. Much interest was aroused locally by these remarks, 
and the writers resolved to investigate the alluvials derived from the 
altered ultrabasic igneous rocks of the Lizard Group. 
The main rock features of the Peninsula may be shortly indicated 
as follows. Against the Ordovician and Devonian schists of 
Cornwall hes tiie Lizard Group of greatly altered igneous and 
sedimentary rocks of the Basement Complex, separated from the 
schists by the Lizard fault and a much brecciated belt which traverses 
the Peninsula from east to west. The lizard Group may be briefly 
described as consisting of mica- and hornblende-schists successively 
intruded by ultrabasic, basic, and acid bosses and dykes, all much 
altered by regional metamorphism: Minor intrusions are common, 
and frequently along the borders of the larger masses mixed rocks 
