Change of Climate during Geological Epochs. 127 



period, following as they really do immediately after the warm 

 coal -period ? 



Passing beyond the coal-period to the earlier age of the Old 

 Red Sandstone, we again meet with the evidence of glacial ac- 

 tion*. 



It was long ago suggested by Agassiz, that the ancient climate 

 was subject to alternate depressions and risings of temperature, 

 coinciding with great destruction and renewal of life. Modern 

 geological investigation seems to favour this conjecture. " Thus, 

 looking," says Mr. Page, "at the Cambrian strata of the 

 northern hemisphere — their angular grits and conglomerates, 

 their extreme paucity of fossil forms, and other features — we 

 are at once reminded of the action of ice and the presence of 

 ungenial conditions. This is followed over the same areas by 

 the more genial and exuberant period of Siluria ; which is in 

 turn succeeded by the Old Red Sandstone, whose grits and 

 bouldery conglomerates, as well as paucity of vegetable forms, 

 once more suggest the recurrence of colder influences. Follow- 

 ing the Old Red, we have the exuberant flora and fauna of the 

 coal-period, again to be succeeded by the scanty life-forms and 

 grits and conglomerates of Permia. Again the trias and oolite 

 of the northern hemisphere are characterized by life-forms that 

 betoken warm and genial conditions ; while the chalk that suc- 

 ceeds imbeds water-worn blocks of granite and lignite, which 

 would seem to imply the presence of ice-drift and deposit in 

 seas that were open to boreal influences. Next the early terti- 

 aries occur over the same areas, marked by plants and animals 

 that indicate a warm and genial climate; and this in turn gives 

 place to the well-known glacial or boulder-drift epoch, once 

 more to be succeeded by the milder influences of the post-ter- 

 tiary or current era " t. 



The principal cause why geologists have been so slow in ad- 

 mitting the existence of cold epochs during the earlier ages of 

 our earth's history, is the idea still entertained by some, that, 

 owing to the influence of internal heat, the climate of our globe 

 was then very much warmer than at present, and that, ever since, 

 it has been gradually growing colder and colder in consequence 

 of the decrease of internal heat. But, as we have already seen, 

 the notion is quite erroneous. If the climate in former ages was 

 warmer than at present, the cause must be sought for elsewhere. 



Some have referred the change of climate to a difference in 

 the distribution of land and sea. It has been supposed by some, 



* See 'The Past and Present Life of the Globe.' By David Page, 

 F.G.S., p. 91 ; and « Advanced Text-Book,' p. 132. 

 t The Past and Present Life of the Globe, p. 190. 



