Reports & Proceedings—Geologists’ Association. 237 
platinum is characteristic of ultrabasic rocks. In the basic intrusions 
there is little or no vein-formation, the ore-bodies being basal or 
marginal, sometimes disseminated more or less uniformly, and 
pneumatolysis plays little or no part in ore-formation. 
GEOLOGISTS’ ASSOCIATION. 
Friday, April 8, 1921. 
“The Influence of Geology and Topography on the Growth of 
London.” By C. E. N. Bromehead, B.A., F.GS. 
A new geological map of London, showing the “ buried streams ”’, 
with short accounts of the streams. The site of London, with reasons 
for its selection. Roman London. The site of Westminster, at the 
first ford. The Roman Roads round London. The growth of London 
and Westminster and the connecting link along the “ Strand”. 
To the end of mediaeval times the necessity for a defensible position 
limited the growth. The great expansion in the seventeenth century 
brought to an end by the question of water supply. The supply 
from-the terrace-gravels and the means taken to supplement it. 
Growth ceases until beginning of nineteenth century. A few of 
the ways in which Geology affects London to-day. 
CORRESPONDENCE. 
GEOLOGY OF THE NINGI HILLS. 
Str,—As Major Williams in his letter in the February number 
handsomely disclaims all intention of being controversial, I am 
content to leave the matter where it now stands. My attitude 
towards the larger questions which Major Williams has raised will 
be made clear in a forthcoming publication of the Geological Survey 
of Nigeria, and I need not, therefore, transgress upon your space. 
May I say, however, that I have no recollection of ever stating— 
and certainly not in my last letter—that tinstone is or can be found 
only in the younger granites. That is quite a different matter from 
the question of its origin. 
J. D. FaLconer. 
JOS, NIGERIA, N.P. 
5th March, 1921. 
‘*LISSONS.”’ 
Sir,—Mr. Upton’s explanation of the continuous rise of air from 
“lissons”’, quoted in the article under the above head by 
Mr. L. Richardson in the March number of the GEOLOGICAL 
Macazinr, does not appear quite satisfactory. Surely the barometric 
pressure into the ground would be able to act more readily in a 
downward direction through the fissures than through the interstices 
of the country rock. 
