CROLL ON CLIMATE AND TIME. 725 



other reasons than those given by Mr. Croll why its effects may be 

 greatly varied. Nor can there be any doubt that changes in the gen- 

 eral level of the land, altering coast-lines and the currents of both the 

 atmosphere and the ocean, are important agents in modifying tempera- 

 tures. We cannot follow into details Mr. Croll's exhaustive inquiries, 

 but our readers will be interested in his answer to the well-known 

 theory of Sir Charles Lyell. According to that eminent authority, a 

 period of polar cold will result from a great increase of elevation and 

 extent of land in polar regions, and a warm period in the polar zones 

 will occur by a great accession of land in the equatorial regions. But 

 Mr. Croll shows that such changes in the distribution of land would 

 be followed by opposite results that a great accession of land in the 

 equatorial zone would destroy the system of ocean-circulation by 

 which the heat of the equator is made to do service in warming the 

 ocean and the air of colder zones. 



In Mr. Croll's theory it is impossible that both hemispheres should 

 be glaciated at the same time. Not only must periods of heat alter- 

 nate with those of cold in one hemisphere, but a glacial epoch in one 

 is accompanied by a temperate epoch in the other. 



It is evident that changes of climate such as are shown to have 

 occurred must have arisen from general, not from any local cause or 

 accidental combination of causes, and if this be so there may yet ap- 

 pear a reliable means of determining not only the amount and extent 

 of the changes, but the periods of their duration and recurrence. 

 The question of time in geological history is an important and cer- 

 tainly a most interesting one, and the interest in it has increased 

 since the announcement by Dr. Tiddeman and others that human relics 

 have been found in deposits of the warm inter-glacial periods. 



Two methods have been adopted by Mr. Croll which are supposed 

 to throw light on this subject. One is to fix the period and duration 

 of the epochs of greatest heat and cold by computing the period and 

 duration of the astronomical coincidences already noticed, by which ac- 

 cording to his theory those epochs were brought about. The other 

 method is applied to estimating the time since the close of the last 

 glacial epoch by changes known to have taken place in the earth's 

 surface, and the general lowering of the land by denudation. To do 

 this, he says, " we have only to ascertain the quantity of sediment 

 annually carried down by the river systems." 



By this means it is found that the lowering may have been nearly 

 a foot in 6,000 years. But when we consider how greatly the general 

 result may have been interfered with by the alternate elevation and 

 depression of the land, a work now going on, we realize that Mr. 

 Croll's conclusions do little more than profoundly impress the mind 

 with the vastness of time required in some of the most obvious of 

 Nature's operations. 



By the first-named method computations have been made extending 



