DISCOVERY OF THE LAND 179 



ness must have been largely absent. There could have been no cold ocean 

 currents flowing beside warm lands to desiccate the winds blowing over 

 them, as in southern California and northern Chile, at the present time. 

 The most characteristic land deposits, those of deserts and ice-sheets, be- 

 long especially to the short periods of stress and trouble separating the 

 long, genial, but unenterprising, geological ages, and hence must be rela- 

 tively rare in the column of formations. 



These comparatively unusual types of deposits began to attract atten- 

 tion about sixty years ago in Europe, and geologists of the Indian Survey 

 correctly interpreted the ancient Talchir boulder-clays in 1859. With 

 deserts before their eyes for comparison, they recognized also ancient arid 

 deposits. In America not much attention was given to continental for- 

 mations till Davis and his brilliant physiographic school, twenty-five years 

 ago, began to explain the Cenozoic beds of the west as dry-land deposits. 

 At about the same time Walther and other Germans took up the careful 

 study of desert processes, giving the clue to the origin of ancient red 

 sandstones and their accompaniments. Of late years most of us have 

 paid at least brief visits to deserts and have felt the charm of their bare- 

 ness, their loneliness, their clear, cool, night skies and hot orange haze 

 at noon, and have watched the dusty pillars of the "go-devils" transport 

 a train-load of dust across the Kalihari, or have seen the low dance of the 

 yellow sand grains as a hot wind builds up a barchan in Nubia. We have 

 seen the selective carving of the desert sand-blast on rocks of unequal 

 hardness, have wondered at the brown desert varnish on exposed rock 

 surfaces, and have speculated as to the origin of "calcrete" or "kankar." 



Geologists are now on the alert for continental, and especially desert, 

 formations, and there are few red sandstones which have not been picked 

 out of the marine ragbag and set aside as belonging to the land. It is 

 even possible that the pendulum has in some cases swung too far and will 

 have to swing back again. Some of the red sandstones or shales handed 

 over to the desert may yet disclose marine fossils and have to return to 

 the seashore. 



A glance through recent text-books of geology in English, French, and 

 German shows how widely attention has been given of late years to conti- 

 nental, and especially desert, formations. Arid conditions have been recog- 

 nized, or at least suspected, in nearly all the main subdivisions of his- 

 torical geology. They have been mentioned by one author or another in 

 the Pleistocene, the Pliocene, the Miocene, the Eocene; the Cretaceous 

 and the Triassic; the Permian, the Carboniferous, the Devonian, the 

 Silurian, and the Cambrian; the Keweenawan, and possibly one or two 

 earlier of the Precambrian series. In fact, only the Jurassic and the 



