Hugh Miller — Formation of Rock- Basins, 451 



discloses. Admitting always the influence of geographical conditions 

 and currents as agents materially modifying climates however caused, 

 it has long appeared to me that, so far as the Glacial Period proper 

 is concerned, it is to the 6th suggested cause, a diminution in the 

 heat emitted by the Sun, that the probabilities incline. I am led to 

 this view, first, because it seems to me that, for the reasons men- 

 tioned in discussing the Geographical theory (No. 4), the cause of 

 this period must have been a cosmical one ; secondly, because, so far 

 as the evidence has jQt been collected on the point, the cold of 

 that period seems to have fallen upon the earth while its axis was in 

 its present position ; and thirdly, because nothing has yet been 

 found to raise a doubt as to the glaciation of the northern and 

 southern hemispheres having been synchronous. As the Glacial 

 Period has passed awaj'', the cause suggested under head No. 1, a 

 decrease in the planet's heat, cannot have been the cause of that 

 Period, even if its influence could be otherwise detected in changes 

 of climate. Of the remaining suggested causes besides No. 6, only 

 Nos. 2 and 7 satisfj'- the three postulates which lead me to refer the 

 Glacial Period to No. 6 ; but No. 7 is obnoxious to the objection of 

 the obscurity which surrounds it ; and No. 2 has to contend with the 

 insufficiency of the limits within which astronomers confine the 

 variation in the obliquity of the ecliptic to produce any material 

 effect on climate, and also with the doubt as to what would be the 

 effect of a large increase or decrease in that obliquity, were the 

 astronomers' objections to its possibility removed. 



How far this 6th suggested cause may have been the agency in 

 producing the climates of geological periods older than the Glacial, 

 I do not venture even to entertain an opinion, beyond this, that* 

 whatever was the cause giving rise to the Glacial Period proper, it 

 is more probable than otherwise that the same cause had a material, 

 though perhaps not an exclusive share in influencing the climates of 

 earlier periods. It may indeed eventually prove to be the case that 

 the close of the Geological record comprises the only part of the 

 earth's history during which any change in climate occurred, beyond 

 what Cause No. 4 was sufficient to produce, the normal temperature 

 of the earth's surface prior to the Pliocene Period having been 

 higher than it now is. 



IV. — Considerations bearing on Theories of the Formation op 



EOCK-BASINS. 

 By Hugh Miller, F.G.S., Assoc. E.S.M., H.M. Geological Survey. 



THE arguments I endeavoured to outline in the June Number of 

 the Geological Magazine have evidently been partly misunder- 

 stood by at least one gentleman whose opinion is valuable, and I 

 desire therefore to illustrate them somewhat further. 



Although Mr. Bonney's criticism^ seems to raise the question. 



What is a tarn ? mere titles are of little importance ; and in order 



to make his remarks relevant, it must be assumed that a rock-basin 



from half a mile to a mile long, and about 20 feet deep, comes 



1 Geol. Mag. for August, 1876, p. 377. 



