Dr. James Croli—Aqueous Vapour and Perpetual Snow. 3857 
The interest attaching to this paper will be very considerably in- 
creased if we view the facts here recapitulated in connexion with the 
theory of mountain-making first enunciated by Hall, and, sub- 
sequently elaborated by the American geologists Dana, Sterry 
Hunt, Le Conte and others. The theory referred to may be briefly 
stated as follows. In the formation of a mountain mass the first 
important operation of which we have any evidence is the accumulation 
of a vast thickness of sedimentary deposits accompanied by a slow 
subsidence of the earth’s crust. At a certain point, owing to the 
weakening of the floor on which the sedimentary deposits were first 
thrown down, the lateral pressure due to the secular cooling of the earth 
operates upon the sedimentary mass, which thereby becomes “ folded, 
profoundly broken, shoved along, fractured and pressed into a narrow 
space” (Dana, Manual of Geology, page 749). ‘To the downward 
bending of the earth’s crust Professor Dana has applied the term 
Geosynclinal, and to the mountain mass which results from the break- 
ing up of the sedimentary deposits formed in. a geosynclinal he has 
applied the term synclinorium. The theory here sketched out appears 
to be applicable to all great mountain ranges. I would extend it still 
further, however, and use it to account for all the great systems of folds 
which are revealed by a study of the structure of the earth’s crust, 
whether these systems occur in mountain ranges or not. Thus it 
seems to me that the four great systems of earth movements referred 
to in the present paper are strictly in accordance with this theory. 
The metamorphosed Pre-Cambrian rocks of N.W. of Scotland, the S.W. 
of Wales and Malvern, represent, it is believed, a great accumulation 
of sedimentary material; the geosynclinal which accompanied the 
formation of this material was broken up before the Cambrian period, 
and in this breaking up the rocks were folded and metamorphosed, 
and the existing strike determined. The lateral pressure acted, as we 
have seen, from the N.E. and 8.W., and therefore, according to our 
theory, the original sedimentation should have been greatest along 
N.W. and S.E. lines. The second great period of sedimentation was 
the Cambrian of Sedgwick, this was broken up in Pre-Silurian 
(Sedgwick) times by pressure from N.W. and 8.E. The maximum 
sedimentation in this case shonld have been in a N.K. and 8.W. 
direction. Thethird great period of sedimentation was brought to a 
close by the formation of our Coal-measures, and this was followed as 
before by folding and contortion. Here, however, we cannot say that 
the folds follow any one course or direction; two parallel courses 
appear to be indicated. The earth movements in the Tertiary period 
followed on sedimentation, which reached its maximum in Central 
Europe, and which culminated in the formation of the Alpine range. 
V.—Agueous Vapour 1n Rexation 10 PerreruaL Snow. 
By James Crott, LL.D., F.R.S. 
OME twelve years ago I gave (Phil. Mag. March, 1867, “ Climate 
and Time,” p. 548) what appears to me to be the true explana- 
tion of that apparently paradoxical fact observed by Mr. Glaisher, 
that the difference of reading between a thermometer exposed to 
