SOME ASPECTS OF GEOLOGIC TIME 259 



tinually re-absorbing carbonates, with the probable evolution of 

 volcanic carbon dioxide. 1 In the second place, Mr. Reade 

 made a very modest estimate of the limestone buried under 

 the ocean. 



Another aspect of the same subject is found in the masses 

 of marine limestone found in the sedimentaries of particular 

 geologic epochs. According to the data of Sir John Murray, 

 there is brought down to the sea each year roughly 2,000 

 million tons of calcium carbonate. This, if evenly deposited 

 over the ocean floor (say 150 millions of square miles), would 

 raise its level to the extent of only a foot in 90,000 years. Sir 

 John Murray has calculated that carbonate deposition is actually 

 taking place over only a third of that area. It therefore follows 

 that, at the present time, under the sea-floor, vast areas of lime- 

 stone are being laid down at the rate of about a foot in 30,000 

 years. We must note that we have here merely the order of the 

 result. The very deepest sediments are formed more slowly, 

 because, in the vaster abysses, the pressure of the water causes 

 the re-solution of the more delicate of the shells of the forameni- 

 ferse which make the bulk of the oceanic lime deposits. On 

 the other hand, local deposits, and particularly coral, are often 

 formed much more rapidly. We must notice, however, that 

 excess in particular places implies that the rate of formation in 

 the ordinary deep sea deposits must be slower by a correspond- 

 ing amount. 



There remains the question whether the vast masses of 

 mountain limestone found in the strata of so many different 

 ages are marine in this sense of the word. Let us, as an 

 example, take the Cretaceous and the Carboniferous deposits. 

 There has been some dispute as to whether these are oceanic, 

 or were formed in shallow water. From the point of view 

 of rapidity of formation, however, it does not greatly matter. 

 What is important for our purpose is whether or no strictly 

 contemporaneous limestone deposits are widespread. Let us, 

 therefore, consider the Carboniferous in greater detail. Early 

 Carboniferous limestone, attaining sometimes to several 

 thousands of feet in thickness, underlies newer rock in nearly 

 all the area of Great Britain. It outcrops in several places, and 

 constitutes the greater part of the bulk of the Mendips. Sir 



1 It has been stated that Mr. Reade overestimated the proportion of limestone. 

 If so, his estimate is liable to a reduction on that account. 



