March 21, 1913] 



SCIENCE 



459 



surface which is at about 750 feet. Section con- 

 tains much water in the limestones, also several 

 feet of salt. Anticlinal theory does not seem to 

 apply; gas is apparently in pockets or lenses of 

 the sand in the calcareous shales. These sand beds 

 are not continuous. Rock pressure varies in dif- 

 ferent wells from 600 to 950 pounds, and flow 

 from 1,000,000 to 5,000,000 cubic feet per day. 



The History of the Bajadas of the Tucson Bolson 



of Arizona: S. S. Visheb. 



The bajada is the long, gentle slope of detrital 

 material at the foot of the mountains in bolsons 

 or areas of centripetal drainage. The bajadas 

 near Tucson have a length of about 10 miles and 

 an average slope of 2° or 100 feet per mile. In 

 spite of a difference of 8° or more in slope be- 

 tween the lower four miles of the canyons and the 

 upper four miles of the bajada, erosion is now 

 taking place on all portions of these thick ter- 

 restrial deposits. The explanation for this wide- 

 spread erosion has been sought by many. The 

 higher portions would, as shown by Salisbury, be 

 eroded as the highlands were lowered, but not the 

 lower portions, nor are certain other features to 

 be thus explained. Over-grazing does not appear 

 to be a sufficient cause. Davis concludes that in 

 maturity the adjustment between one bolson and 

 an adjacent lower one may result in the terracing 

 in the higher. Changes of climate have been ad- 

 vocated: Barren considers that bajadas were 

 formed during the various glacial epochs and have 

 since been destroyed; Huntington believes bajadas 

 were formed in the interglacial epochs, of ma- 

 terial which accumulated in the mountains during 

 the glacial epochs. The present terracing he cor- 

 relates with a recent increase of precipitation. 

 The latter apparently opposing theories for ba- 

 jada formation are in this region seemingly both 

 essentially correct — Barrell's of the bajadas sur- 

 rounding the lower, barren, warmer ranges, and 

 Huntington's those of the higher, forested, cooler 

 mountains upon which glacial conditions were ap- 

 proached since the bajadas of the former are more 

 gentle, have a smaller percentage of clay and are 

 more extensively eroded than those of the larger 

 ranges. The frequency of freezing temperatures 

 are believed to be important. At Tucson a decrease 

 of 10° F. would increase the frequency about 200 

 per cent, and a 20° decrease would result directly 

 in frost about 250 nights in the year. A different 

 distribution of the precipitation would greatly 

 effect the vegetal covering. The percentages of 

 clay and boulder formation would fluctuate ac- 



cordingly as would also accumulation and trans- 

 portation. The Tucson bajadas appear to have 

 been mainly formed at a time when the average 

 temperatures were 10° or more lower; when the 

 precipitation was greater and more uniform — 

 either chiefly late in the glacial epochs (smaller 

 ranges) or at their close (larger ranges). The 

 present terracing is perhaps due to an increase in 

 temperature and a different distribution and 

 amount of rainfall abetted by the advancement 

 of the area in the geographic cycle. 

 The delation of the Lime CreeTc Shales to the 



Cedar Valley Limestones of Floyd County, 



Iowa: A. O. Thomas. 



The Devonian system of Iowa is represented by 

 sediments belonging to two series, the Middle and 

 the Upper Devonian. The stages of the Upper 

 Devonian do not overlie each other, but each is 

 locally developed and geographically separate. 

 Moreover, each lies uneonformably on the Cedar 

 Valley stage of the Middle Devonian. Field study 

 in Floyd County on the areal distribution and 

 geological relations of the Lime Creek stage of 

 the Upper Devonian has demonstrated a wide- 

 spread unconformity at its base. A new substage, 

 for which the name Nora limestone is proposed, 

 is added as the lowest member of the Lime Creek 

 stage. 

 A Four Mile Section along the Missouri River 



South of Columbia, Missouri: E. B. Branson. 



Strata are well exposed along the bluff of the 

 Missouri Eiver and are, in general, nearly hori- 

 zontal. Eight distinct and well-exposed uncon- 

 formities occur within four miles. 

 The Belation of Geological Activity to Conserva- 

 tion of Soil and the Waters of Flowing 



Streams: Luella Agnes Owen. 



A view of the advance of geology as a science, 

 and of early geological research, shows the grow- 

 ing appreciation of the all-important power of 

 water in the development and progress of conti- 

 nents as well as their destruction or partial de- 

 nudation for continual rebuilding. All atmos- 

 pheric forces unite their energy with that of the 

 flowing streams in every land to work without rest 

 in tearing down the high places of the earth and 

 transporting the waste for renewal of valleys and 

 building new coast lines. So, the present geolog- 

 ical epoch is preeminently the Age of Rivers, and 

 in necessary works of conservation man may 

 change the application of natural law to meet his 

 needs and pleasure, but the law itself is un- 



