92 



SCIENCE 



[N. S. Vol. XL VI. No. 1178 



deposition by the wind, one may at least ten- 

 tatively conclude that the climate of this 

 region was rather arid at the time the sands 

 composing these rocks were put in place by 

 the forces of nature. This part of the con- 

 tinent was evidently a portion of the great 

 inland desert which is thought to have existed 

 in Triassic times. 



It seems probable that at one season this 

 particular locality was swept by winds carry- 

 ing a burden of well-worn quartz grains, 

 which was dropped when the force of the 

 wind was checked. When the wind rose 

 again, some of this sand was doubtless moved 

 farther on, but a little remained to add to the 

 accumulating layers beneath. At another 

 season, the surface of this wind-laid sand was 

 covered by a deposit of entirely diilerent ma- 

 terial, probably brought from some neighbor- 

 ing zone of alluviation by torrential rains. 

 When the water had iiowed on, or evaporated, 

 the red-brown material became exposed to the 

 winds, part of it was doubtless swept away, 

 but some was covered with desert sand which 

 continued to accumulate until the next 

 freshet sent more of the red-brown sediment 

 into the depression in the zone of dimes. 

 That this was approximately the mode of 

 deposition seems likely, when we find the one 

 layer to be characteristically wind-borne, and 

 the other water-borne, when all the accom- 

 panying facts are considered, and comparison 



is made with sand deposits that are being 

 formed at the present time. 



The study of this sandstone takes on an 

 added interest if we note further that the 

 frequency of recurrence of the brown or white 

 layers often shows a striking regularity or 

 periodicity. Where we find fairly broad 

 white bands, with very thin brown layers 

 alternating, it would seem that a relatively 

 dry season is indicated. On the other hand, 

 when the brown layers are very numerous and 

 close together, it apparently points to fre- 

 quent rains, with comparatively little deposi- 

 tion of the white sands by the wind. In the 

 solid rock wall, as observed in the quarries, 

 one can note the more or less regular recur- 

 rence of the wider bands of white, and if one 

 could be sure that here a wide white band 

 and one or more narrow brown bands repre- 

 sented the deposit of an arid year, one could 

 determine the time required to produce a 

 given thickness of this rock and also draw 

 some conclusion as to the relative aridity of 

 a given year or a series of years. But one 

 can not at present state, beyond reasonable 

 limits, the amounts of either kind of material 

 that might be deposited in a year, and there- 

 fore one may not yet say definitely how long 

 it took for a given stratum to be formed, or 

 whether the aridity indicated by a white band 

 corresponds to one season or to several. It 

 may be interesting to note, however, that the 

 recurrence of groups of brown layers with a 



