6 



[No. 1. 



by from 1 to 5 per cent of the mean annual value, according as 

 the winter may occur in aphelion or in perihelion, and according 

 to the greater or lesser eccentricity of the Earth's orbit. 



Such an alteration in the velocity of the ocean currents 

 will produce an alternation in the climate, which, it is true, may 

 not be very considerable, but will still be sufficiently great to 

 leave its traces on the strata. During cold and dry periods, the 

 rivers are in a great measure nourished by springs. That water 

 has filtered slowly through the strata and is rich in soluble sub- 

 stances, but its small volume and gentle velocity causes it to 

 carry with it less clay, sand and gravel. During rainy periods 

 the rain-water washes a mass of such substances down, but it 

 passes quickly away, and as it flows principally on the surface 

 it does not have sufficient time to dissolve much. The springs 

 also flow more freely during rains, but their waters become more 

 mixed with the surface rain-water. The rivers become, thus, 

 proportionally poorer in soluble matters, but have a greater 

 volume, and the accelerated current carries with it more clay, 

 sand, and gravel into the estuaries. The dry periods will there- 

 fore be relatively richer in purely chemical deposits taken up 

 in the clearer waters, and the rainy periods in mechanical de- 

 posits. Layers of both kinds will of course be formed at all 

 times, but they will be deposited in different places according 

 to the variable amount of rainfall. I therefore conclude that when 

 thick beds of alluvial sand and clay alternate, when soft clay- 

 and marl alternate with hard marl and lime, when thickish lay- 

 ers of loose sand alternate with sand-stone which is cemented 

 with chemically deposited cements (iron, silicic acid, lime), when 

 clay alternates with layers of septaria etc. then do the first- 

 named beds, at any rate, date from periods with warmer ocean 

 waters and greater rainfall; that is the same as to say, as re- 

 gards Western Europe, from periods with winter in aphelion. 



That this alternation of strata is due to a period of several 

 thousands of years duration, may be gathered from this fact, that 

 the fossils vary quickly upwards through the beds. In the Ter- 

 tiary formation only few, often only 4 to 5 such alternations of 



