1879.] 



Limestone as an Index of Geological Time. 



281 



V. " Limestone as an Index of Geological Time." By T. 

 Mellard Reade, CLE., F.G.S., F.R.I.B.A. Communicated 

 by A. C. Ramsay, D.C.L., F.R.S., Director-General of the 

 Geological Survey of the United Kingdom. Received 

 December 24, 1878. 



(Abstract.) 



The geological history of the globe is written only in its sedi- 

 mentary strata, but if we trace its history backwards, nnless we 

 assume absolute uniformity, we arrive at a time when the first sedi- 

 ments resulted from the degradation of the original crust of the 

 globe. 



There is no known rock to which a geologist could point and say 

 " that is the material from which all sedimentary rocks have been 

 derived," but analogy leads us to suppose that if the earth had an 

 igneous origin, the original materials upon which the elements first 

 began to work were of the nature of granite or basalt. 



From a variety of considerations drawn from borings, mines, faults, 

 natural gorges and proved thicknesses of the strata of certain moun- 

 tain chains, the author arrives at the conclusion that the sedimentary 

 crust of the earth is at least of an average actual thickness of one 

 mile, and infers from the proportionate amount of carbonates and 

 sulphates of lime to materials in suspension in various river waters 

 flowing from a variety of formations, that one-tenth of the thickness 

 of this crust is calcareous. 



Limestone rocks have been, geology tells us, in process of formation 

 from the earliest known ages, but the extensive series of analyses of 

 water made by Dr. Frankland for the Rivers Pollution Commission, 

 shows that the later strata in Great Britain are much more calcareous 

 than the earlier. The same holds true of the continent of Europe, 

 and the balance of evidence seems in favour of the supposition that 

 there has been on the whole a gradual progressive increase or evolu- 

 tion of lime. The " Challenger " soundings show that carbonate of 

 lime in the form of tests of organisms is a general deposit character- 

 ising the greater part of the ocean bottoms, while the materials in 

 suspension are, excepting in the case of transport by ice, deposited 

 within a distance of 200 miles of land. 



This wider distribution in space of lime, the author thinks, must also 

 profoundly influence its distribution in time, and he shows this by 

 example and illustration. It can also be proved to demonstration 

 that the greater part of the ocean bottom must at one time or another 



VOL. XXVIII. x 



