OCTOBEK 31, 1919] 



SCIENCE 



413 



OROGENICS OF THE GREAT BASIN 



When, about forty years ago, the members 

 of the Fortieth Parallel Survey leisurely trav- 

 ersed the mile-high Great Basin between the 

 lofty Eockies and the Sierra ISTevada, their 

 impressions on the configuration of the moun- 

 tain ranges were that there was chiefly folding 

 of the Appalachian type wherein the synclines 

 were deeply and almost completely filled with 

 illy transported rock-waste from the neighbor- 

 ing highlands. 



A decade later, corps of other governmental 

 surveys, passing through this part of the coim- 

 try, put an entirely different interpretation on 

 the origin of the rugged desert ranges. Novel 

 as well as brilliant was the conception that 

 these mountains were recently tilted fault- 

 blocks of gigantic size. This fancy led to 

 another brilliant idea — the hypothesis of iso- 

 statie compensation, whereby there is ready 

 response to the transference of eroded rock 

 materials from one point to another, loading 

 areas sinking and unloading areas rising. 



Curiously enough when the isostatic hy- 

 pothesis came to be critically tested in the 

 field no faults bounding the orographic blocks 

 were discoverable. More than a third of a 

 century passed since the idea was first pro- 

 mulgated and yet no one appeared to find im- 

 peachable evidences of the alleged crustal 

 ruptures. A governmental expedition, espe- 

 cially fitted out to solve the problem and 

 headed by the author of the hypothesis him- 

 self, failad to establish the claim or to publish 

 the desired data. Many of the so-called fault- 

 scarps which were described as marking the 

 basin ranges proved not to be fault phenomena 

 at all, but merely characteristic features of 

 normal eolic erosion at the level of maximum 

 activity. Other investigators searching espe- 

 cially for the assumed faults found not major 

 lines of this character bounding the present 

 mountains but instead discovered dislocation 

 phenomena in the most unexpected situations 

 — far out on the smooth intermontane plains. 

 There was manifestly no genetic relationships 

 existing between mountain profile and geologic 

 structure. On the whole the proposed hy- 



pothesis of Basin Range structure proved to 

 be singularly unsupported by observation. 



In the meanwhile testimony of another kind 

 abundantly accumulated bearing upon the 

 problem. When the presence of faulting was 

 all but completely discredited, it was shown 

 that although extensive rupturing actually 

 occurred in the region it was mainly relatively 

 ancient. It long antedated the time when the 

 present mountains took form. 



There is, however, a third possible explana- 

 tion of the faulting phenomena displayed in 

 the Great Basin. The major faulting of the 

 region may not be of the normal gravity type 

 as is so commonly supposed. It may be 

 mainly of overthrust character. In support of 

 this suggestion, as a general proposition, there 

 are a number of considerations besides the 

 almost conclusive theoretical one. A remark- 

 able circumstance is that some of these thrust- 

 planes in tlie desert ranges are often mistaken 

 for lines of unconformities. This may be the 

 real reason why so few normal faults are 

 found bounding the mountain blocks. The 

 overpowering influence of the normal fault 

 idea has much to do with the general misin- 

 terpretation of Basin Range orogeny. 



Concerning the tectonic genesis of the 

 desert ranges we shall now probably have to 

 give up our brilliant conceptions of mountain 

 blocks floating on the liquid interior of the 

 earth much after the fashion of ice-cakes in a 

 river at time of spring break-up. What the 

 substitute shall be may not yet be perfectly 

 clear. Mountains of circumdenudation through 

 the differential activity of eolic erosion under 

 the stimulus of the aridity and over a region 

 previously effected widely by overthrust move- 

 ments seems more nearly in accordance with 

 the larger aspects of the conditions presented. 

 At least the extent of the overthrust activity 

 is worthy of the most careful consideration 

 and severest test in the field. 



Chaeles Keyes 



distribution of the fresh-water 



medusa, craspedacusta, in the 



united states 



In Science, November 8, 1907, the writer 

 published a brief account of the appearance 



