1886. J OX VAHIATIONS OF CLIMATE IX THE COURSE OF TIME. 15 



because it would not then liave left behind siich great ramparts, 

 but the sand and the gravel would have been spread more evenly. 

 During the melting, however, its edge remained at time statio- 

 nary. or advanced perhaps a little. At each such event a ro w 

 of moraines was formed, and as the same row stretches across large 

 tracts of the country, they cannot be attributed to local circum- 

 stances, but we have to assume that periodical variations of the 

 climate were the cause of the manner in ivhich the ice receded. 



We found in the peat bogs alternately layers of different 

 kinds, peat alternating with remains of forests several times, 

 and we saw ho w this was easiest explained by periods of change 

 in the climate, but these alternating layers are not peculiar to 

 the peat alone, but found in all stratified formations, loose as 

 well as solid, whether deposited in fresh or salt water, or on land, 

 in all the strata from the Laurentian gneiss to the loose deposits 

 of the present age. Take a geological structure from any age, 

 alternating layers will be found everywhere. Sand alternates 

 with gravel, sandstone with conglomerate, clay with sand, slate 

 with sand or sandstone, marl with clay, chalk with marl, and 

 so on. The layers vary in thickness from several yards to less 

 than an inch. 



The solid rock withers away by the action of air and water 

 in heat and cold, partly it crumbles away mechanically and 

 partly it changes chemically. The products of the erosion are 

 carried by wind or running water as dust, in dissolved or origi- 

 nal state, and deposited in places more or less remote from those 

 where they were produced. The foaming mountain stream carries 

 often great stones in its course, and the softer the wind the 

 weaker the current the tiner is the matter deposited. When 

 the current becomes weak the gravel sinks first, then the sand, 

 then the clay, and, finally, the chemically dissolved lime by the 

 animal life in the water. When we, therefore, have a change 

 of la3'ers of different composition through all geological ages, 

 as those mentioned above, it must be due to the circumstance 

 that the speed of the depositing stream was always varying, 

 now increasing, now decreasing. 



