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, mudstones, 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 incredibly 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 1 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, II 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 America 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 atmosphere and hydrosphere during the weathering processes. Accordingly, Leith and Mead have shown that the cubical 5 Leith, C. K., and Mead, W. J., "Metamorphic Geology," 1915, pp. 59-97.

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, 32 per cent of sandstones, 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 permanently 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 1 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 1 mile, and

FIG. 9.-Generalized map of North America in Paleozoic time, showing in white the lands or positive elements, in darkest shading the three geosynclines or subpositive elements, and in lightest shading the extensive neutral medial area of the continent. The two heavy black lines indicate the Cincinnati and Ozark (Kankakee) uplifts. From the Pirsson-Schuchert "Text-book of Geology," published by John Wiley & Sons, Inc.

that over the remaining small portion they vary from 1 to 20 miles; yet the greater depths do not cover more than oneeighth of the continent, and they occur in the long and narrow troughs of sedimentation, of which the two best known lie in the Appalachian and Rocky Mountain regions (see Fig. 9).

Eras of geological time. Now let us see how geologists divide this great pile of at least 53 miles of sediments and their included fossils. (A synopsis of geologic time is presented on page 46, and in Fig. 3.) The first era, or Archeozoic time, with the oldest known strata, has no proved fossils, though a very low form of water-living alga appears to have been present. Even if we exclude the probable fossil evidence, the presence in Ontario of 18 miles of sediments, of which about one-half is impure limestone along with much graphite, indicates unmistakably that life was already in existence. As we are treating of the oldest known time in the history of the earth, this is the place to emphasize what results were then attained and what natural processes were at work.

At the very beginning of the Archeozoic era, the earth had a rocky and fairly stable exterior known as the lithosphere, an atmosphere and a hydrosphere; protruding continents, and oceanic basins that were filled with slightly saline water; twicedaily tides, wear and tear of the ocean waves against the lands, reduction of the high lands by the atmosphere, and, through the rains, washing of the soil and solution materials into the seas and oceans. The sun shone then as now and made life possible, and the man in the moon was as clearly developed as he is today, remaining so ever since because this satellite has had neither atmosphere nor water to wash away his face. We therefore see that the earth was then very much as it is now, with these great differences, that at first the atmosphere was almost devoid of free oxygen and probably richer in carbon dioxide, that the lands were not covered with verdure, and that what there was of life was of a very low

order and possibly more prevalent on the lands than in the

oceans.

Then follows the second era of geologic time, the Proterozoic, with a thickness in south-central Canada of nearly 14 miles of coarse sediments almost devoid of fossils, and 4 miles of lavas. Probably more than three-fourths of this mass is of fresh-water and volcanic origin, and less than one-fourth may be of marine origin. In the Rocky Mountains there are upwards of 7 miles of less coarse sediments that include about 1.5 miles of possibly marine limestones. In this connection it should also be noted that one of the remarkable recent discoveries in geology is that we know very little of marine Proterozoic sediments (in the sense of marine and fossiliferous Paleozoic formations) and that there is as yet even no hint as to where geologists are to look for them.

The Archeozoic and Proterozoic eras have the most ancient strata of the earth and their combined thickness is 32 miles as against about 21 miles for all subsequent, fossiliferous strata. In this we discern the astounding fact that at least one-half of geological time and the greater part of the earth's history had passed before organisms became sufficiently endowed with hard parts to be abundantly preserved as fossils in the sedimentary rocks. It also means that the evolution of life for an incredibly long time was exceedingly slow in rising from the simple unicellular forms into the multicellular forms of plants and animals. It may be that the Archeozoic was entirely occupied in originating only the unicellular plants and animals. In the Proterozoic, organic differentiation appears to have gone on faster, because at the very beginning of the Paleozoic era are found all of the main kinds of marine animals other than fishes. The subject of the origin and progress of life from the unicellular forms to the most complex modern vertebrates will be developed by Professors Woodruff and Lull in subsequent lectures.