258 SCIENCE PROGRESS 



chemical structure of the Earth's crust. Of the geochemical 

 methods, the best so far discovered is probably that based on 

 calculations concerning the amount of limestone in the rocks of 

 the Earth. As is well known, limestone rock is not, and cannot 

 be, a part of the Earth's original crust. It has been slowly dis- 

 solved out of the primitive and the newer igneous rocks, carried 

 to the sea in solution, and there used by the various marine 

 organisms for the formation of their shells. These minute shells 

 have either formed comparatively rapid local concretions of coral 

 reef, or have gathered, at a rate inconceivably slow, in the 

 abysses of the ocean. Geologic time must have been great 

 enough to admit of the removal of all this substance from its 

 place of origin and its deposition in the conditions where we 

 now find it. 



The geologist whose name is most intimately associated with 

 the question of the evolution of carbonate of lime is the late 

 Mr. Mellard Reade. 1 Mr. Reade did not attempt to fix any 

 actual figures. He did not think the subject was ripe for such 

 exactitude ; but he maintained strongly that these data proved 

 that the Earth had existed for a much longer period than the 

 mathematical physicist of his time had thought to be possible. 

 The results of the Challenger expedition have enabled us, 

 within a reasonable degree of accuracy, to map out the character 

 of the ocean floor. In the neighbourhood of land, the sediments 

 are, in the main, composed of detritus from the rivers. In the 

 greatest depths, the carbonate redissolves and the floor is 

 composed of " red clay." Between these two limits, the main 

 covering of the ocean floor is carbonate of lime. 



Mr. Mellard Reade made deductions from the calculated 

 amount of carbonate of lime, and the time that it would take for 

 this to be evolved from igneous rock. From that amount, he 

 inferred that the process must have been going on for at least 

 600,000,000 years. This calculation I believe to be substantially 

 sound, though the details will require revision in the light of 

 more recent knowledge. It is, I believe, possible to assert a 

 probable minimum of the order of 500,000,000 of years. The 

 number is a minimum for two reasons. In the first place, 

 igneous action, whether at the surface or deep-seated, is con- 



1 See various papers in the Geological Magazine, also papers read to the 

 Geological Society. A most important pamphlet is republished under the title of 

 Chemical Denudation . 



