258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1916. 



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 contained no marine fossils. The ancient bowlder 

 clays were merely coarse, water-formed deposits of some peculiar 

 kind. 



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

 deposits of any sort, but by the gaps in the sequence of formations, 

 for normal land conditions mean erosion and denudation. Their 

 only record is usually a discordance, and a dry-land interval shown 

 only by an unconformity naturally passed almost unnoticed. Most 

 of the chapters of the world's history are written under water and 

 show a strong bias toward the side of the w^ater animals. 



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

 ditions which have a good chance of being preserved and recognized 

 are those of the coal swamps, and they persist mainly because they 

 are on debatable ground often invaded b}'^ the sea. During much 

 the greater part of the world's history happenings on the land are 

 recorded only in the most accidental way, as by some stray leaf or 

 tree trunk or carcass drifting down 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 condi- 

 tions. 



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 were mountainous 

 or flat we can only infer from the kind of sediments they sent down 

 to the sea. 



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

 imiild and moist, even to the poles, and deserts and ice sheets were 

 apparently absent. We are living in an exceptional time character- 

 ized by extremes of climate and are apt to think of such extremes as 

 normal. When Miocene plane trees grew luxuriantly on Spitzber- 

 gen, in latitude 78°, the whole circulatory system of air and water 

 must have been different from the one we are accustomed to. Ex- 

 tremes of cold and perhaps also of dryness must have been largely 

 absent. There could have been no cold ocean currents flowing beside 

 warm lands to desiccate the winds blowing over them, as in southern 

 California and northern Chile, at the present time. The most char- 

 acteristic land deposits, those of deserts and ice sheets, belong espe- 

 cially to the short periods of stress and trouble separating the long, 

 genial, but unenterprising, geological ages, and hence must be rela- 

 tively rare in the column of formations. 



These comparatively unusual types of deposits began to attract at- 

 tention about 60 years ago in Europe, and geologists of the Indian 

 survey correctly interpreted the ancient Talchir bowlder-clays in 1859. 



