2 6o SCIENCE PROGRESS 



Archibald Geikie states that a continuous formation can be 

 traced over 750 English miles from the Western headlands 

 of Ireland into the heart of Europe. How far it extends, or 

 once extended, under what is now the Atlantic, and its extreme 

 limits north and south do not appear to have been determined. 

 Contemporaneous limestone (though interstratified with coal 

 beds) is stated to be found in Scotland, Silesia, Central and 

 Southern Europe, Spain, and the Urals. Limestone of the same 

 era is found in China, in the Central Himalayas, in Morocco, 

 Algeria, and other parts of Africa, and also in Australia. In 

 America, early Carboniferous (Mississippian) limestone (in some 

 places mixed with sedimentary) underlies a large portion of the 

 United States. It is 5,000 feet thick in the Canadian Rockies, 

 and is extensively developed in Alaska. The known area of the 

 formation must be reckoned in millions of square miles. If we 

 add to this an estimate for countries as yet geologically un- 

 explored, for that which is now under the ocean, for that which 

 has been eroded in the vast period which has elapsed since early 

 Carboniferous times, there can be no doubt that it was deposited 

 under oceanic conditions. For the essential point is the area 

 and thickness of the formation. If we can reckon the area of 

 contemporaneous limestone at many millions of square miles, 

 the current controversy whether it was deposited under deep or 

 under shallow water conditions becomes of small importance for 

 the purposes of our argument. Under no circumstances is it 

 possible for the ocean, which contains an infinitesimal pro- 

 portion of carbonate of lime, to deposit, for any prolonged 

 period, more than is brought down by the rivers to the sea. 

 Let us, therefore, assume ordinary marine conditions, and assess 

 the probable average thickness of early Carboniferous limestone 

 at the very low estimate of 1,000 feet, and let us allow as the 

 probable rapidity of formation three times the present average, 

 a foot in ten thousand years, we thereby obtain a minimum of 

 ten million years for only a portion of a recognised geologic 

 epoch. Such figures as it is possible to give are, of course, very 

 crude guess-work, and no stress is laid on them, but they will 

 serve to point out a useful line of research. 



The only important query which is likely to be raised, and 

 which, indeed, has been raised, is whether, in past times, the 

 proportion of carbon dioxide in the atmosphere might not have 

 been excessive, and so the amount of carbonate carried to the 



