Notices of Memoirs — Limestone an Index of Time. 549 



Fig. 7. Neritopsis Morrisianus, H. Woochv. Clay-marl, Tertiary, loc. cit. 



,, 8. Melania subfossilis, H. Woochv. Tertiary, „ 

 „ 9. „ rivularis ? Philippi. „ 



„ 10. „ ? (cast of). White Tertiary Clay-marl, „ 



,, 11. ,, pyramis, Benson. Tertiary, „ 



,, 12. ,, stcblactea, H. Woochv. ,, ,, 



« 13. ,, costata, var. glabra, H. Woochv. Tertiary, „ 



„ 14. Dentalium, sp. Tertiary Clay-marl, ?> 



itotices oif 1 imiieimiohrs.. 



I. — " Limestone as an Index of Geological Time." By T. 

 Mellakd Eeade, C.B., F.G.S. [From the Proceedings of the 

 Koyal Society, No. 192, 1879.] 



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

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

 mountain chains, the author arrives at the conclusion that the sedi- 

 mentary 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 in process of formation from the 

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

 made by Dr. Frankland for. the Eivers 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 evolution of 

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

 the form of tests of organisms is a general deposit characterizing 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 

 have been land, else the rocks of the continents would have become 

 gradually less, instead of more, calcareous. 



Thus the arguments drawn from the geographical distribution of 

 animals are reinforced by physical considerations. 



