GEOLOGY. 239 



intercalated within the sandstones certain sheets of trap, mainly intru- 

 sive ; and using these distinctive beds as data planes in the otherwise 

 homogeneous deposits, he finds that the same beds re appear many 

 times, and that in some cases several successive trap ridges are formed 

 by outcropping edges of a single sheet, the mass having been thrown 

 into a series of parallel blocks and subsequently so degraded as to leave 

 the harder trap projecting in the form of strongly accented surface 

 features. Ilis explanation of the faulting in this region is unique. He 

 supposes that the Triassic sandstones were originally deposited in hor- 

 izontal beds upon an eroded surface of highly inclined (but not vertical) 

 schists, gueises, etc. ; that after the completion of Triassic deposition, 

 horizontal compression occurred ; that the inclined crystalline strata 

 slipped upon each other, as does a row of fallen books when ijushed to 

 upright position, and thus became more nearly vertical ; and that as 

 the successive blocks (defined perhaps by intercalations of softer mat- 

 ter) approached verticality the veneer of Triassic sediments above was 

 broken through by a succession of approximately vertical faults coin- 

 ciding with the planes of slipping among the crystalline strata. Davis's 

 hypothesis is certainly suggestive, and, if valid, constitutes a notable 

 advance in the branch of geology dealing with the deformation of the 

 earth. 



The terra incognita of American geographers for many years has 

 been, singularly enough, not the comparatively inaccessible mountains 

 and deserts of the West, but a tract in southwestern Missouri, north- 

 western Arkansas, and eastern Indian Territory. Somewhere here was 

 supposed to belong that will-o'-the-wisp of geographers and geologists 

 alike, the Ozark Mountain system — a half-ideal mountanic tract com- 

 monly named in geographic treatises and sometimes vaguely located in 

 small-scale maps, though no geographer knew their exact position and 

 no geologist knew their structure. Now during the last year or two 

 a part of this tract has been surveyed topographically by oflicers of the 

 U. S. Geological Survey and its general configuration ascertained ; and 

 moreover the work of the geological survey of Arkansas has extended 

 along its southern flanks and the predominant structural characteristics 

 ascertained. According to Branner and Coinstock, the region suffered 

 post-Paleozoic deformation now expressed by corrugation approaching 

 the Appalachian type, the strata lying in a series of folds of nearly 

 east- west direction ; * and as pointed out by the former in a communica- 

 tion before the American Association for the xVdvancement of Science 

 at Cleveland, the crystalline rocks found along the southern flanks of 

 the corrugated tract are not eruptive, as they have hitherto been re- 

 garded, but Arch;ean, so that the region would appear to be homologous 

 with and probably a continuation of the Appalachian region of eastern 

 United States. 



*Ark. Geol. Survey^ Ann. Rpt. 1888, vol. i, inapa, and p. xxx. 



