INVESTIGATION OF TEMPERATUEE CHANGES. 219 



been furnished, can be obtained of any such decrease having taken place 

 during the geological ages, either during their entire duration, or for an}' 

 portion of it. Such an inquiry will naturally connect itself with an investi- 

 gation into the changes of climate which may have taken place on the earth, 

 however caused and wherever prevailing, and will likewise lead to the dis- 

 cussion of the various and somewhat numerous theories of climatic change 

 which have been promulgated by different wTiters on the subject of "geologi- 

 cal climates." Moreover, our attention will be turned, more decidedly than 

 it has thus far been, to the vertical element in the orographic changes which 

 have taken place on the earth's surface, and especially with reference to the 

 causes giving rise to precipitation in the form of snow rather than in that of 

 rain. A portion of this review and discussion will be brought forward in the 

 following section ; a part of it will, on the other hand, find its most natural 

 place in connection with that which is to be given in the chapter devoted 

 to the Glacial epoch. 



Sectiox VI. — Examination of the Evidences of former Changes of Temperature 

 on the Eaiih. Has the Solar Radiation been diminishing in Intensity during all or 

 any Portion of the Geological Periods ? 



As has been shown in the preceding pages, the proofs of desiccation 

 during the later geological periods, over various portions of the earth's 

 surface, are ample. After the facts bearing on this question had been laid 

 before the reader, it was explained, in the course of a discussion on the 

 causes influencing precipitation, that there were two prominent ones which 

 would tend to a diminution of its quantity; namely, an increase in the 

 amount of land surface on the globe, and a decrease in the intensity of solar 

 radiation. It was farther shown that the first-mentioned of these causes had 

 — as was, at least, extremely probable — been in action during the whole 

 time of the accumulation of the sedimentary formations, the continental 

 masses having been growing larger as geological time passed on. 



It now remains to inquire whether it is also true, or highly probable, that 

 there has been a tendency to a decrease of precipitation through the agency 

 of the other specified cause, namely, a diminution in the intensity of the 

 sun's heat. 



The term "geological climates" is one under which are commonly classed 

 those real or supposed variations of the earth's climatic condition which, 



