408 A. Bljtt on the probable Cause of 



but they contain more water, and their more powerful current 

 carries more clay, sand, and gravel into the basin. Hence 

 the drier seasons will be richer in purely chemical deposits, 

 which will be transported in the clearer water ; the wet 

 seasons in mechanical deposits. Strata of both kinds are 

 formed, of course, at all times, but they are deposited at 

 different places in accordance with the variation in the 

 quantity of rain. Thus, I assume that when thick deposits 

 of river-sand and clay alternate with each other, when soft 

 clay and marl alternate with hard marl or limestone, when 

 thick strata of loose sand alternate with sandstone, which is 

 bound together by chemically produced cement (iron, silica, 

 lime), when clay alternates with Septaria-beds &c., then, in 

 each case, the first-named deposit shows itself to belong to 

 seasons with a warmer sea and a greater quantity of rain, 

 which, as regards Western Europe, will mean seasons with 

 the winter in aphelion. 



That this alternation of deposits implies a period of several 

 thousand years' duration is shown by the fact that the fossils 

 change rapidly through the strata. In the Tertiary forma- 

 tions there are only a few, often only 4-5, such changes of 

 deposits in each stage. The whole Oligocene period has only 

 about 30, the Miocene still fewer, and the Pliocene barely 20 

 such changes. 



In this way, in my opinion, the precessions stamp them- 

 selves upon the strata, and this should therefore furnish a 

 means of measuring time. The greater the eccentricity of the 

 orbit, the more strongly marked M'ill the periods be ; when 

 the orbit approaches the circular form, they are less 

 recognizable*. 



Eeferring for other things to the two memoirs cited and to 

 my paper "On Variations of the Weather in the course of Time" 

 {Letterstedtske Nordisk Tidskrift, 1885, in English in Forh. 

 Vid. Selsk. i Christiania, 188H, No. 8) I will pass on to 

 examine whether there is any probable ground for supposing 

 that the other proposition is also correct, whether it is con- 



* But the perihelion also shifts to aud fro. The time between two 

 aphelia in the winter solstice varied thus in postglacial times by fully 

 4600 years. This must have some influence. The longer a period with 

 winters in aphelion lasts the longer will the warm currents in the Atlantic 

 increase in strength, and the greater will be the changes of climate. The 

 mild period during which Bergenian sea-animals lived in the Christiania 

 Pjord. and which has left its traces elsewhere in our hemisphere, was, in 

 my opinion, a consequence of such an unusually long period with the 

 winter in aphelion. The winter solstice fell in aphelion (according to 

 Croll) 61,300, 33,300, and 11,700 years ago. The middle of the Atlantic 

 period with Bergenian sea-animals in the Christiania Fjord fell, from the 

 testimony of the peat-mosses, 33-34,000 years ago, therefore in accordance 

 with the peyiod of 28,000 years. 



