238 Correspondence—J. G. Hamling. 
The following alternative explanation is suggested. The normal 
temperature lapse rate in the lower layers of the atmosphere is 
—1° F. per 300 feet, whilst the adiabatic lapse rate is —1° F. per 
172 feet. When the lapse rate in the atmosphere exceeds the latter 
quantity, a condition of unstable equilibrium ensues, and convection 
immediately sets in to restore equilibrium. Now, assuming the 
temperature gradient in the Oolites at Stroud to be — 1° F. per 60 feet 
measured in the same direction as that of the atmosphere, i.e. in 
an upward direction ; and further assuming that the air in a fissure 
remains sufficiently long in contact with the country rock to take 
up its temperature (a period of a few minutes would suffice), it would 
appear that the, column of air in the fissure would be in an 
exceedingly unstable condition and would at once tend to rise to the 
surface. This effect should be most noticeable during the winter, 
when no inversion in the lapse rate is likely to occur in the subsoil, 
as is the case in the summer. 
No doubt the barometric tendency materially affects the velocity 
of the current —a negative tendency (i.e. when pressure is decreasing) 
should cause an increase; by analogy with the effect produced on 
fire-damp evolution in coal mines on such occasions. 
The velocity of the current might temporarily be increased during 
the winter season by the rise in the level of the water-table subsequent 
to heavy rains, causing the upward displacement of air. 
Norman L. SILVESTER. 
_ BEDFORD COLLEGE, REGENT’S PARK, N.W. 
April, 1921. . 
CORRELATION OF THE MEADFOOT AND SIEGENIAN BEDS. 
S1r,—I read with interest,in the April number of the GkoLogicaL 
Macazine Dr. HE. Asselberghs’ remarks on the above subject. On 
p. 167 he says :— 
“Later, other fossiliferous localities were discovered. Several 
lists of fossils have been published, but a great number of the 
identifications, based only on the already old descriptions of Phillips 
and Davidson, are doubtful.” 
He then quotes from a list of mine (Guo. Mac., 1909, pp. 133-4) 
as an instance. I should like to say I did not venture to identify 
any of the fossils given in my list. I submitted them first to Upfield 
Green (who was working with me at the time) and afterwards to 
W. A. H. Ussher. They were carefully examined by both, as was 
my list before publication. I gather from subsequent remarks that 
Dr. Asselberghs has confidence in the opinion of these authorities. 
Referring to the Devonian rocks of North Devon, Dr. Asselberghs 
says: “The stratigraphical succession is less known than in the 
South.” I was not aware of this. 
J. G. Hamuine. 
THE CLOSE, BARNSTAPLE. 
