February 22, 1918] 



SCIENCE 



193 



they should furnish sufficient grounds to 

 justify changing tlie BI^A, but if on the other 

 hand they can not, or rather will not, then it 

 follows that they sliould use the terms as they 

 stand in the BXA. 



If we are willing to stand so firmly in the 

 principles of evolution and if we recognize 

 man as but another animal in the long series, 

 it seems entirely justifiable to use the same 

 terms throughout for homologous structures, 

 and certainly for the general space positions 

 and orientations. 



Perhaps an open discussion of the matter 

 might be of some interest and effect; con- 

 cessions might be made on both sides which 

 will give happy results. 



Thomas Byrd Magath 



College of Medicine of the 

 University of Illinois, 

 Chicago, III. 



rate of desert delta growth 

 From the presence and position of the allu- 

 vial fans which so conspicuously mark some 

 of the old shore-lines of ancient Lake Bonne- 

 ville, that gigantic precursor of the Great Salt 

 Lake of Utah, it is inferred that during the 

 long period of desiccation which that vast 

 water-body underwent the lowering of the lake 

 level did not take place uniformly but experi- 

 enced more or less protracted pauses. The 

 most notable of these supposed halts in the re- 

 cession of the waters is thought to be repre- 

 sented by the embankment denominated the 

 Provo Beach. 



The Provo terrace, which lies about 600 feet 

 above the surface of the present Great Salt 

 Lake, and 400 feet below the highest water 

 stage of the ancient lake, is distinguished by 

 extensive alluvial cones which are commonly 

 regarded as true delta deposits. The great 

 magnitude of some of these deltas is interpreted 

 as furnishing conclusive evidence of long tarry- 

 ing of the old lake waters at this level. 



That the unusual size of the Provo deltas is 

 not a necessary consequence of long lagging of 

 lake waters at this stage seems demonstrated 

 by recent extensive observations that enable 

 quantitative calculations to be made of the 



actual time occupied in desert delta formation. 

 The possible rapidity proves to be very much 

 beyond all ordinary expectations. Concerning 

 the formation of the Provo deltas there are 

 several accelerating factors which do not ob- 

 tain in the normal desert fans of alluvium that 

 so often collect on the piedmonts at the mouths 

 of canyons. These are the great volumes of 

 nearby morainic materials which filled the val- 

 leys of the Wasatch and other ranges, the 

 presence of a convenient water-body in which 

 to concentrate the debris washed out of the 

 canyons, and the position of the Provo level on 

 the line where plain meets mountain. 



The alluvial fans characterizing the pied- 

 monts of many desert ranges are usually small, 

 owing largely to the fact no doubt that the 

 mountains have little soil material or coarse 

 rock-waste. In some instances the alluvial 

 debris forms merely a thin veneer over a low 

 cone of the rocky subsrtructure. The out-wash 

 of boulders and pebbles serves rather as a pro- 

 tection against the general lowering of the 

 plains surface through eolian erosion. Not 

 a few of the desert fans have thus really a rock 

 floor just as have the interment plains them- 

 selves, and are not, strictly speaking, alluvial 

 cones at all. 



In strong contrast are the desert fans some- 

 times produced by normal water action. Two 

 instances in particular may be cited in illus- 

 tration of the actual rapidity with which the 

 process sometimes goes on. Near Ivanpah, in 

 southeastern California, a shallow trench was 

 once dug diagonally down a sloping bajada belt 

 in order to protect a railroad grade from pos- 

 sible wash of sporadic rains. Soon a cloud- 

 burst happened to come. In an hour's time a 

 great gully 75 feet deep, 50 feet wide and sev- 

 eral miles long was excavated in the soft soil. 

 The bulk of the dirt was redeposited at the foot 

 of the sloping plain in a broad fan of more 

 than a mile radius. In another instance, near 

 Socorro, New Mexico, the bank of an arroyo 

 was cut to take care of future storm-waters. 

 In a single night this spillway was deepened to 

 50 feet and an alluvial cone nearly 100 feet 

 high and nearly three miles radius was 

 formed. 



