Correspondence—Mr. W. H. L. Monch. 187 
derived from Mona, or the Isle of Anglesey, and the several parts 
are distinguished as the Holyhead group, or Lower Monian, the 
St. David’s group and the equivalent of the South-Stack Series, or 
Middle Monian, and the Bray-Head group, or Upper Monian. 
The ‘‘ Pebidian” represents the St. David’s group, and but for its 
termination, which indicates a system, might be used as an alterna- 
tive. The ‘ Dimetian and Arvonian” are intrusive granites, or 
felsitic flows associated with the same group. 
The Monian system, though much metamorphosed in its lowest 
parts, is not considered Archean, but as a lower sedimentary system 
than the Cambrian, and hence the lowest system of our ordinary 
stratified rocks. 
CORRESPONDENCE, 
—.—_—_. 
THE CAUSES OF THE GLACIAL AND MILD PERIODS. 
Str,—There is a theory on the above subject (due to M. Poisson, 
I believe) which seems to me worthy of more attention than it has 
recently received from geologists, viz. that the earth with the 
entire solar system has travelled through hotter and colder regions 
of space at different periods. Hotter and colder regions of course 
mean regions in which it received more or less heat from the stars. 
Now that the solar system is moving with considerable velocity 
through space may be regarded as an established fact, and though 
it may not have materially changed its position among the stars 
in historical time, it is otherwise with regard to the much longer 
geological periods. Mr. Maxwell Hall indeed recently computed 
that the solar system is moving round a distant centre in a period of 
18 or 14 millions of years, though his data I think must be regarded 
as uncertain. Further, every astronomer knows that there are richer 
and poorer regions among the stars, the passage of the solar system 
through which would materially affect the amount of stellar heat. 
But it may be thought that the total amount of stellar heat is 
insignificant compared with solar heat. The observations of Pouillet 
and Herschel however point to an opposite conclusion. ‘The mean 
temperature of the earth is more than 500° F. above the absolute 
zero, of which according to them not more than 800° F. is due to 
solar radiation. The remaining 200° F. must be ascribed to the 
heat of the stars. An increase or diminution of 10 per cent. in this 
would raise or lower the mean temperature of the earth by 20° F., 
and such an increase confined to the Northern sky would make the 
North hemisphere 20° F. hotter than the Southern. 
I am aware that the results arrived at by Pouillet and Herschel 
are doubted by many. Further experiments on the same subject by 
skilled physicists are much to be desired. And should these ex- 
periments result in showing that the total amount of stellar heat is 
insignificant, this fact would not be altogether favourable to the 
theory of Dr. Croll. If the solar heat is competent to raise the 
mean temperature of the earth by 500° F. instead of 300°F., the 
