:>56 The Amen'cioi Geolo(ii\sl. April, 1895 
cause important fluctuations oi" tlic ice-sheets in both Ameri- 
ca and Europe. 
Finding CrolPs tlieory unsound, ^Ir. Culverwcll atlvances 
one of his own, which is stat^tl as follows: 
Is it possible lluit there may liavc heeii any considerable iiitercliaiiyes 
of atmosjiliere bi'twecMi tlie earth ami tlie regioiisof space through wiiicli 
il lias passed'; 11 is eerlain that there must ha\e been some such iiiter- 
cliaiii;!'. 'W'helher the atmospheric pressure is iiicreasiiii:- or dimiiiisli- 
iiiu' depends on wlielliei-, in the course ot' the earth's motion throu.n'h 
si)ace. more of till' interstelhir molecules yet entangled in the earth's at- 
mosphere ti\an. leaving' that almosphere, u-etcnian.uled in the interstel- 
lar iiases through which the earth happens to be ])assing, and are Ilius 
dragged away from the earth. If once we were allowed to assume the 
magnitu(h' of these changes, the \vln)le ditticulty of e.\i)laining (ilaciul 
()r genial ages, so far as temperature changes are concerned, would van- 
ish. For instance, if due to some gaseous conditions of space, or per- 
haps lo the alisorption into the atmosphere of the gaseous components 
of meteoriti's or shooting stars, there be an addition to the atmosi)heric 
pressure of one millimetre in three cent ui'ies, and if this [)rocess has 
been conliiiued for "JO.dUO or ■*.">, 000 years, then it follows that some 25,- 
000 years ago the atmospheric pressure would have been less by about 
one-tenth part than it is at present. This would be equivalent to rais- 
ing land and sea by about 2,.')0() feet, for the blanketing effect of the de- 
creased atmosjjhere woidd be about the same as that which now lies 
al)ove a mountain "i.-lOO feet high. Thus a Glacial e|>ocli might be 
produced iritliiiiit iinij ultirdtioii ht tlo' (jeDgiuntliiatl eondltiniis. And if. on 
the other hand, either through the earth plunging into a more gaseati'd 
region <if space, or through some catastrophe, the atmt)sphei'ic i)ressure 
were to l)e much increased, the resulting increaseof tempcu-ature might 
be very great, and a genial age might hv the result — a riseof50°F. might 
rea<lily be got If two bodies tiying about in space come into 
collision they may generate a mass of gas. and if this ma.ss of gas hap- 
pens to get in the earth's path it may be caught \\\^. Indeed, if we as- 
sume tlial the earth's atmosphere maintained a strict ecpiilibrium with 
the interstellar molecules at, say, a distance of 1,000 miles from the 
earth's surface, then a doubling of the almost infinitesimal pressure there 
would necessitate a doubling of the pressure at the earth's surface. Of 
course this su|)position is only used for the purpttse of illiislraling the 
fact that a small alteration of interstellar pressure, if spread over a suf- 
ficiently vast space, might eventually give rise lo a considerable change 
in the almosi)here the problem of atmospheric interchanges is so 
complicated that 1 may at all events ht)pe to enjoy my hy[)olhesis for a 
considerable time before anyone succeed.s in giving it a really decisive 
overthrow. Its great advantage us Ji theory of the (ilacial epoch is. 
that it does not require geographical changes such as are usuall\' postu- 
lated in connection with all other theories, even the Astronomical one.* 
*Geol. Magazine, IV, vol. ii, pp. 64, 6;i, Feb., 181)5. 
