178 A. P. COLEMAN DllY LAND IN GEOLOGY 



beck, with its araucarian stumps still rooted in the soil, was occasionally 

 recognized, though such occurrences are almost unknown in formations 

 older than the Carboniferous. It was recognized, also, that heat and 

 drought best accounted for the beds of gypsum and rock-salt found in 

 several of the more ancient formations, though the materials might have 

 come from the evaporation of inclosed arms of the sea, and so might not 

 be really continental deposits. 



The most typical land deposits, those of arid and of glacial climates, 

 were seldom recognized as such and were generally included among the 

 marine stratified rocks, though the absence of fossils was disquieting. 

 Even the red sandstones, with their hot, desert colors, were often looked 

 on as marine, or else possibly as formed in great lakes, because they con- 

 tained no marine fossils. The ancient boulder-clays were merely coarse, 

 water-formed deposits of some peculiar kind. 



In most cases, however, dry-land periods are not represented by de- 

 posits of any sort, but by the gaps in the sequence of formations, for 

 normal land conditions mean erosion and denudation. Tbeir only record 

 is usually a discordance, and a dry-land interval shown only by an uncon- 

 formity naturally passed almost unnoticed. Most of the cbapters of the 

 world's history are Avritten under water and show a strong bias toward 

 the side of the Avater animals. 



The only continental deposits beside those of arid and glacial condi- 

 tions which have a good chance of being preserved and recognized are 

 those of the coal SAvamps, and they persist mainly because they are on 

 debatable ground often invaded by the sea. During much the greater 

 part of the world's history happenings on the land are recorded only in 

 the most accidental Avay, as by some stray leaf or tree trunk or carcass 

 drifting doAvn a river to be buried in the mud at its mouth. It is seldom 

 that land formations can be found on a broad enough scale to reconstruct 

 continental surfaces and conditions. 



Though it is certain that lands and their inhabitants have existed in 

 unbroken succession from early times, the lands themselves are in geology 

 mostly shadowy things. Whether they Avere mountainous or flat Ave can 

 only infer from the kind of sediments they sent cIoaati to the sea. 



During most of the world's history the climate seems to have been mild 

 and moist, even to the poles, and deserts and ice-sheets Avere apparently 

 absent. We are living in an exceptional time characterized by extremes 

 of climate and are apt to think of such extremes as normal. When Mio- 

 cene plane trees grew luxuriantly on Spitzbergen, in latitude 78°, the 

 Avhole circulatory system of air and water must have been different from 

 the one Ave are accustomed to. Extremes of cold and perhaps also of dry- 



