278 Reviews—A. Blytt—Displacement of Beach-lines. 
nations of strata, formed under different physical conditions, could 
not, as a rule, be attributed to local phenomena, but are rather due 
to general and more permanent causes. 
Thus in cold and dry periods, the rivers which are nourished 
mainly by springs, are rich in soluble substances, and from their 
comparatively small volume, they carry little detrital matter. In 
rainy periods, the rivers, then poor in soluble matter, transport 
much clay, sand, and gravel, Hence dry periods should be marked by 
chemical deposits ; rainy periods by mechanically-formed sediment. 
Admitting that layers formed chemically and mechanically will 
be accumulated at all times, the author observes that they will be 
deposited in different places according to the variable amount of 
rainfall. When, however, clay aera with limestone; when 
thick beds of sand alternate with sandstone cemented chemically by 
salts of iron and lime, or by silica; when clay alternates with layers 
of septaria, ete. : then (in the author's opinion) do the first-named 
beds date from periods with warmer ocean-waters and greater rain- 
fall, than attend the formation of the latter. 
The author is indeed unfortunate in his examples. If he had 
omitted all mention of septaria, which were formed since the deposi- 
tion of the strata in which they are embedded, and which frequently 
enclose organisms identical with those found in the clay above and 
below them ; and if he had taken no notice of sands which may be 
hardened by chemical agency long subsequent to original deposition, 
and which indeed are compacted at various horizons—then indeed 
we might pass on to consider his general conclusions with less 
misgiving than we do now; but our faith in his geological know- 
ledge is sorely tried. 
Other alternations of strata are considered to be due to “a rise 
and fall of the sea in relation to the land,” the result being ‘‘an 
alternation of deep-sea formations with littoral formations or fresh- 
water beds.” 
Of the two explanations thus given of alternations of strata, both 
causes in the author’s opinion are periodically active. In the one 
case climatic causes are repeated at somewhat regular intervals; in 
the other, the changes in the relative level of land and water are 
repeated at longer and less regular intervals. 
Periodical variations of climate, explained by comparatively small 
fluctuations in the extremes of temperature and rainfall, are 
attributed to periodical changes in the strength of ocean-currents. 
From ocean-currents the author turns to prevailing winds, to changes 
in atmospheric pressure, and finally to the precession of the equinoxes 
and the eccentricity of the earth’s orbit as originating periodical 
variations of climate. 
Thus alternately in the northern and southern hemispheres, 
the precession of the equinoxes causes the summer to be, for about 
10,500 years, longer than the winter, but in the subsequent 10,500 
years to be shorter. The author considers that although the 
periodical changes of climate were not great, yet they were 
sufficient to imprint themselves on the strata, in the alternation of 
als 
”~— a. ere 
