60 EVOLUTION OF THE EARTH 



by the ancient oceans. Out of all these widely scattered 

 records the geologist pieces together the geological column, 

 embodying the history of the earth, and yet at best we seem 

 to have recovered far less than half of it. The record may 

 be complete in the oceanic basins, but here it is forever hidden 

 from hammer and mind. However, if we piece together all of 

 the thicker known geological formations of sandstones, mud- 

 stones, and limestones into a superposed sequence, we get a 

 pile of about 53 miles in thickness as a mean estimate, with 

 the maximum thicknesses attaining to over 67 miles (Sollas 

 gives a maximum of 63.5 miles). This means the more or 

 less rapid wearing away almost to sea-level, one after another, 

 of more than twenty ranges of mountains like the present 

 European Alps or the American Rockies. During the in- 

 credibly long intermediate times, when the lands were planed 

 to a low relief, there was very little erosion. 



Ratios of muds, sandstones, and limestones. We have seen 

 that the continents have lost through atmospheric erosion a 

 layer of igneous rock between I and 2 miles thick in the 

 course of geologic time. This amount of eroded average 

 igneous rock should theoretically resolve itself into 30 per cent 

 of solution materials and 70 per cent of detritals, or 80 per 

 cent of mudstones, 1 1 per cent of sandstones, and 9 per cent 

 of limestones. Holmes gives 70 per cent of shales (20 per 

 cent quartz), 16 per cent of sandstones (75 per cent quartz), 

 and 14 per cent limestones (75 per cent calcium carbonate). 

 However, actual observations of the stratified rocks of Amer- 

 ica and Europe show quite different percentages for these three 

 categories of water-laid sediments, and the difference is due 

 to a great increase of pore-space in the fragmented materials 

 and to the addition of material extracted out of the atmos- 

 phere and hydrosphere during the weathering processes. 

 Accordingly, Leith and Mead 5 have shown that the cubical 



5 Leith, C. K., and Mead, W. J., "Metamorphic Geology," 1915, pp. 59-97. 



