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Limestone as an Index of Geological Time. [Jan. 23, 



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. 



The author goes on to show that the area of granitic and volcanic 

 rocks in Europe and the part of Asia between the Caspian and the 

 Black Sea, as shown in Murchison's map of Europe, is two-twenty- 

 fifths (-2%) of the whole ; much of this is probably remelfced sediments 

 and some of the granites the product of metamorphism. 



From considerations stated at length it is estimated that the area 

 of exposures of igneous to sedimentary rocks would be for all geo- 

 logical time liberally averaged at one- tenth (^q) of the whole. 



These igneous rocks are either the original materials of the globe 

 protruded upwards, or they are melted sediments or a mixture of the 

 two. 



The only igneous rocks we know of are of the nature of granites 

 and traps. If these rocks do not constitute the substratum of the 

 earth, and all known rocks, igneous as well as sedimentary, are deriva- 

 tive, either geological time is infinite, or the rock from which they 

 are derived is, so far as we know, annihilated geologically speaking, 

 and we have no records of it left. 



If we assume the latter as true, the past is immeasurable, but 

 in order to arrive at a minimum age of the earth, the author starts 

 from the hypothesis that the fundamental rocks were granitic and 

 trappean. 



From eighteen analyses by Dr. Frankland, it is shown that the water 

 flowing from granitic and igneous rock districts in Great Britain con- 

 tains on an average 3*73 parts per 100,000 of sulphates and carbonates 

 of lime. 



The amount of water that runs off the ground is given for several 

 of the great continental river basins in Europe, Asia, 'Africa, and 

 America. The annual depth of rain running off the granitic and 

 igneous rock areas, taking into consideration the greater height at 

 which they usually lie and the possibility of greater rainfall in earlier 

 ages, is averaged at 28 inches, aud the annual contribution of lime in 

 solution in the forms of carbonates and sulphates at 70 tons per 

 square mile. 



With these elements, and giving due weight to certain physical 

 considerations that have been urged in limitation of the earth's age, 

 the author proceeds to his calculations, arriving at this result, that 

 the elimination of the calcareons matter contained in the sedimentary 

 crust of the earth must have occupied at least 600 millions of years. 

 The actual time occupied in the formation of the groups of strata as 

 divided into relative ages by Professor Ramsay, is inferred as 

 follows : — 



