DRY LAND IN GEOLOGY COLEMAN. 259 



With deserts before their eyes for comparison, they recognized also 

 ancient arid deposits. In America not much attention was given to 

 continental formations till Davis and his brilliant physiographic 

 school, 25 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 bareness, 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 textbooks of geology in English, French, 

 and German shows how widely attention has been given of late years 

 to continental, and especially desert, formations. Arid conditions 

 have been recognized, or at least suspected, in nearly all the main 

 subdivisions of historical 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 Car- 

 boniferous, the Devonian, the Silurian, and the Cambrian; the 

 Keweenawan, and possibly one or two earlier of the pre-Cambrian 

 series. In fact, only the Jurassic and the Ordovician seem to have 

 escaped the drought, and it may be that a more careful search through 

 the literature would disclose deserts there also. 



A number of the suggestions noted are only tentative, however, and 

 wide-spread and unmistakable desert formations seem confined to 

 the Pleistocene, Triassic, Permian, Devonian, and late pre-Cambrian. 

 Of these the Pleistocene deserts may be looked on as continuing to the 

 present, the Triassic deserts form an aftermath of the arid conditions 

 of the Permian, and the Devonian deserts seem less extensive than 

 the others. The three times of greatest aridity appear to be : (1) The 

 Pleistocene continuing to the present ; (2) the Permian-Triassic ; (3) 

 the late pre-Cambrian. 



