BIOLOGICAL CHANGE IN GEOLOGICAL TIME. 



97 



sediment in laminae of current bedding, and to separate the grains 

 of sand according to their relative densities."* 



The testimony of the Torridon Sandstone is repeated by every 

 succeeding formation, and so we may estimate geological time by 

 them and get a conception of it by the witness of a few. The 

 Old Red Sandstone of Herefordshire, 10,000 feet thick, is an 

 accumulation of grains of quartz and clay, derived from the 

 surface of older rocks. The Carboniferous Limestone of England 

 has a thickness, visible to any visitor to Clifton, of 5,000 feet, all 

 of carbonate of lime extracted from sea-water by marine living 

 forms which has received its solution from the land. Our English 

 Chalk is over 1,000 feet thick and occupies thousands of square 

 miles after very great extensions have been removed, and the 

 whole of this vast mass of carbonate of lime has been formed by 

 the accumulation of minute shells and their more minute frag- 

 ments all produced j-by microscopic animals. A single foot of 

 thickness of this wonderful deposit, the work of countless genera- 

 tions of myriads of microscopic animals, would require fully 1,000 

 years for its accumulation, givin,g; at least a million years for the 

 formation of the Chalk alone. The great Nummulitic Limestone 

 we see in France, in Egypt, and as far east as China, has a thick- 

 ness in the south of France of 3,000 feet all similarly accumulated. 

 The Nagelfiuhe of the Rigi in Switzerland, is an accumulation of 

 water-worn pebbles, all rounded fragments of hard rockS; of 

 5,000 feet in thickness. In Asia, too, the still newer Pliocene 

 deposits of the Punjab of India, attain the enormous thickness 

 of 14,000 feet. All these, and many other vast deposits, were 

 accumulated not contemporaneously but during quite different 

 periods of geological time. 



The mean rate of surface erosion to produce the detritus given 

 to the sea at the present time by six representative rivers, the 

 Po, Hoang Ho, Rhone, Ganges, Yang tse kiang, Mississippi, and 

 the Danube, is 1 foot in 3,090 years, or xoVo- ^f a foot in one 

 year. 



When such facts as these are duly weighed it will, I think, 

 be admitted that geologists have very good grounds for estima- 

 ting geological time at a minimum of 100 millions of years. 



The attempt made some years ago on physical grounds to 

 reduce this estimate has now lost its force through the discovery 

 of radio-active bodies, which are potential givers of renewed 

 heat to the earth and the sun. Professor Darwin, now Sir 

 George Darwin, showed that the assumption of the permanency 



Geikie, Text-Booh of Geology, p. 76. 



