114 THE HABITAT OF THE EURYPTERIDA 



the materials which were blown about in one of the earliest deserts 

 recorded in the history of the rocks. This desert differed markedly 

 from all the large ones which are known to us at present, in having 

 a predominance of carbonates instead of silicates in the "sand" 

 grains. We must not, however, push the doctrine of uniformitari- 

 anism too far and insist that all the deserts in the past must have been 

 composed of siliceous grains, because that is the rule in modern large 

 deserts. On a small scale limestone deserts are forming now, and if 

 large areas of limestones could be exposed in the arid regions of Africa 

 or Arabia these limestone deserts would form on a vast scale. But 

 there is now too much diversity in the rocks of the earth's crust, be- 

 cause throughout most of the world the continents have in large 

 part been above sea level during the Tertiary and Quaternary, and 

 erosion has been going on so that many types of rocks are exposed and 

 particularly large areas of crystallines, and when any or all of these 

 are brought under arid climatic conditions, grains of a great range in 

 composition are exposed to the sorting action of wind. In the Middle 

 Siluric of North America, on the other hand, a land area which had 

 been covered by limestone was subjected to arid conditions, and there 

 is no escape from the fact that dominantly lime grains were formed by 

 the prolonged exposure during which mechanical processes alone were 

 active, and decomposition played no part. 



Succeeding the arid or semi-arid climatic conditions of the Salina 

 was a period of greater rainfall and of expansion of the epicontinen- 

 tal seas. The rivers became permanent in response to the rains of a 

 pluvial climate, and there followed upon the period of rock destruc- 

 tion in situ a period of transportation of material from the land into 

 the sea. The prolonged disintegration of the limestones and dolo- 

 mites with local shales had provided a vast soil covering which must 

 have extended to a considerable depth, and which, because of fineness 

 and friability could easily be removed by streams. Even the weak- 

 est little rivulet would be able to carry a small load of this material, 

 which was so conveniently prepared. With the increased moisture in 

 the air decay became active in further breaking down the mechanic- 

 ally disintegrated rocks, and in this way the igneous rocks that were 

 exposed through erosion would yield a certain amount of silica and 

 alumina as would also the shale bands in the limestones. Thus, 

 while the rivers carried material which was dominantly calcareous or 

 magnesian, certain impurities were also included. Some difficulty 

 has been offered by the high amount of alumina, to account for 



