AND ITS INHABITANTS 61 



content of average igneous rocks thus increases in volume by 

 at least 28 per cent, and that on the continents we appear to 

 have 48 per cent of mudstones, 12 per cent of sandstones T and 

 20 per cent of limestones. This marked difference between 

 chemical theory and actual presence is further explained by 

 the unknown quantitative loss from the continents of the finest 

 muds, sands, and solvent materials that the lands have per- 

 manently contributed to the oceanic basins. On the basis of 

 the circulation of radium, Holmes estimates that the loss from 

 the lands to the deep-sea deposits is about one-thirtieth of all 

 the material eroded, i.e., about 300 million tons of the 9,000 

 million tons annually. 



Distribution of the sedimentary rocks. We have seen that, 

 according to theory, all of the continents have lost a layer of 

 original igneous rock that is on the average between i and 

 2 miles in thickness, and it is this material, removed and 

 reworked time and again by the weathering processes and the 

 further chemical and assorting agencies of the rivers and 

 oceans, that has gone into building the known geological 

 column, with its maximum summation thickness of 67 miles. 

 In no one place, however, can be seen more than a small 

 part of this record, for usually the local thickness is under i 

 mile, though there are limited regions where as much as 20 

 miles of it is present. This is because the deposition of the 

 geologic formations in the periodically rising and flooding 

 oceans, as described later in this lecture, takes place at any 

 given time in very limited areas, most commonly, as at 

 present, along the margins of the continents, or in long and 

 narrow troughs the geosynclines (see Fig. 9) within the 

 continental borders, and only periodically over the wider inner 

 portions of the lands. We may add that over the surface of 

 one-third of North America the rocks are all of igneous or 

 crystalline character, that over more than one-half of it the 

 thicknesses of the sedimentary rocks are under I mile, and 



