LITHOGEXESIS OF THE SEDIMENTS 165 



man and others. There can be no doubt that the additional evi- 

 dence furnished by petrographie study as to the composition 

 and structure of the ancient sediments will aid greatly m in- 

 terpreting the conditions of their deposition as well as the nature 

 of their source. Sorby s showed the possibilities in this line 

 several years ago, by his petrographie examination of clays 

 and shales. He found the structure of these to differ greatly, a 

 fact which argues for their formation under very different con- 

 ditions. That such characteristics are fairly constant for any 

 given formation is suggested by the experience of Denckmann 

 who found that a widely distributed Silurian formation of 

 Silesia possessed distinct, petrographie peculiarities by means 

 of which he was able to identify it at those localities wher*» fos- 

 sils were either rare or entirely wanting 9 . It seems certain that 

 to some extent at least, the nature and constitution of the sedi- 

 ments of any given formation are directly related to the climatic 

 conditions which existed during deposition as well as to the 

 source from w T hich they were derived. If then we may determine 

 in. what way climatic changes are registered in the sediments 

 by converging all lines of evidence we shall be able to decipher 

 more accurately by means of the microscope the climates of the 

 past as well as the nature of the ancient lands. Some steps 

 have already been taken in this direction by Mackie 10 who has 

 suggested that the kinds and degree of freshness of the feldspar 

 grains in sandstones may be used as a key in determining the 

 climatic conditions under w T hich the sandstones were formed, 

 and who has demonstrated also that the nature of the parent 

 rock is indicated by the kinds of minerals present and by the 

 nature of their inclusions. 



It is believed that studies of this type will go a long way 

 toward solving the problem of the origin of certain little un- 

 derstood formations such as the red beds and the Coal Measures 

 in addition to furnishing more accurate data regarding the 

 geography of the past. When all these things are better known 

 we shall have the basis also for a much more complete classifica- 

 tion of the sedimentary rocks than the one which we now pos- 

 sess. 



s Quart. Jour. Geol. Soc. Vol. 64. 190S, pp. 171-233. 

 "Cited by Andree, Geol. Rund., Vol. 2, l'.Ul, p. 61. 

 10 Trans. Geol. Soc. Edinburgh, Vol. 7, pp. 443-46S. 



