234 CLARKE— SOME GEOCHEMICAL STATISTICS. [April 2c. 



In order to prevent misapprehension it may be well to reiterate 

 the statement that these estimates of the marine sediments are neces- 

 sarily crude, and represent orders of magnitude only. With better 

 evidence, better estimates may at some future time be made, but 

 accuracy in them is unattainable. The other figures given, for the 

 composition of the igneous rocks and of the ocean are probably near 

 the truth but are still subject to revision and improvement. 



