184 THE POPULAR SCIENCE MONTHLY. 



of climate which we thus anticipate, but, if we mistake not, in contin- 

 ually-lessening force and extent the farther back we go. For, long 

 ages previous to the recent glacial epoch, through all the Tertiary era, 

 the fossil plants and animals indicate the prevalence of a warm and 

 genial climate over the greater part of the globe. Then come the 

 chalk-beds of the Cretaceous period, in which are frequently found 

 water-worn blocks of granite and aggregations of pebbles, proving 

 that then, as now, the iceberg floated down from the north over seas 

 that were quietly depositing the chalk-shells. Still older is found a 

 long series of secondary strata, the Oolite, the Lias, and the Trias, 

 which were deposited in at least sub-tropical climates. They are the 

 burial-grounds of the enormous saurian reptiles that once had an age 

 all to themselves in the world's chronology. Their remains have 

 been found within a thousand miles of the north-pole, thus proving 

 that warm seas covered every zone. 



Between the great divisions of Secondary and Primary in geology, 

 there lies a stratum found only in the higher half of the latitudes, and 

 known as the Permian or New Red Sandstone. The scanty life-forms 

 found in it, and the coarse grit and angular bowlders of which it is 

 composed, evince the well-known glacial action. Geologists generally 

 think that there elapsed between these great divisions a very long 

 period of time in which, excepting the sandstone, but little was done 

 one way or another to build up the crust of the earth or leave a mark 

 in its records. This doubtless indicates periods of very small eccen- 

 tricity. Such periods did occur, according to Mr. Croll's calculations, 

 immediately before and after the great eccentricity of 850,000 years 

 ago, in which we may perhaps conjecture the New Red Sandstone to 

 have been formed. 



Previous to this age were the long Carboniferous periods, during 

 all of which a warm and moist climate prevailed over all lands that 

 have yet been explored. Below the coal-measures are found again 

 the grits and bowldery conglomerates of the Old Red Sandstone, 

 which, with great paucity of organic remains, would imply the alter- 

 nations of somewhat glacial climates. The Silurian, Cambrian, and 

 Laurentian systems preceded the Old Red in the order named, and 

 reach back to the dawn of life on the earth. These formations are 

 of vast thickness, and were deposited at the bottom of warm seas in 

 all parts of the world. 



It cannot be denied that, as we go back in the geologic records, 

 we find more and more the evidences of greater heat and a more 

 equable climate. It is certain that the astronomical relations which 

 we have pointed out the revolutions of the orbital points and the 

 alternations of great and small eccentricity have never ceased to 

 exist. Therefore, if the world had been subjected to only the same 

 solar heat in ancient as in recent periods, there must have been re- 

 peated glacial epochs ; and we should find the bowlder, and the un- 



