June 16, 1905.] 



SCIENCE. 



917 



where they form flat, lateral enrichments of 

 such veins. 



2. Large flat bodies of silica and barite 

 with silver-bearing ores associated with 

 minute supplying fissures. 



3. Deposits of gold-bearing ore composed of 

 an intimate mixture of pyrite and magnetite 

 with actinolite, quartz, epidote and other 

 minerals of supposed contact origin. This 

 class of deposit carries low values in gold and 

 is thought to have been deposited by replace- 

 ment together with the associated minerals by 

 circulating waters subsequent to the porphyry 

 intrusions. 



The geological age of all of these ores is 

 Post Eocene. 



Structure of the Great Plains and the Moun- 

 tains on their Western Margin: N. H. 

 Darton. 



With this communication there was pre- 

 sented an illustration showing the configura- 

 tion of the Dakota sandstone under the Great 

 Plains and on the flanks of the uplifts west- 

 ward. This widespread formation has been 

 extensively explored in its underground dis- 

 tribution, by numerous deep wells, and its 

 outcrop area has been mapped so that the 

 structure of much of the region which it 

 underlies is ascertained. This structure was 

 shown by 100- and 500-foot contour lines and 

 it exhibits many notable features. In gen- 

 eral, under the Great Plains, the formations 

 have but little dip and wide areas are mono- 

 clinal. The uplifts along the mountain border 

 and in southeastern Colorado are marked 

 features and it has been discovered that there 

 is a low anticline extending across north-cen- 

 tral Kansas and western Nebraska nearly 

 to the Black Hills. In the bottom of the 

 basins about Denver and northeast and north- 

 west of the Black Hills the Dakota sandstone 

 lies below sea level. In eastern South Dakota 

 it abuts against the Sioux quartzite and is 

 overlapped by Benton formation. 



Two diagrams were exhibited, illustrating 

 the configuration of the Black Hills and Big- 

 horn Mountain uplifts by contour lines drawn 

 at the surface of Minnekahta limestone in the 

 former and Bighorn limestone in the latter. 



In the central area of these uplifts, where the 

 sedimentary beds have been removed by ero- 

 sion, hj-pothetical contours are given. Both 

 these uplifts are of the ' Uintah type,' steep- 

 sided and flat-topped and evidently due to 

 direct upward pressure and not to crustal con- 

 traction. Profound but local faults along the 

 east side of the Bighorn Mountains are 

 notable features, due mainly to local uplift in 

 Laramie time. 



Fault Phenomena Near Glen Echo, Md.: G. 

 K. Gilbert. 



The locality is a disused quarry on the 

 north bank of the Chesapeake and Ohio Canal, 

 about one fourth mile east of Glen Echo. The 

 rock is gneiss. It is traversed by numerous 

 systems of joints, as many as twenty having 

 been noted. These are inclined in various 

 directions and at various angles. .The joints 

 of each system are approximately parallel, 

 with interspaces ranging from a few feet to 

 at least several yards. The joint surfaces 

 most broadly exposed are not true planes, but 

 show curvature. Many of the joints are evi- 

 dently surfaces of slipping, or fault planes, 

 the observed dislocations ranging from a frac- 

 tion of an inch to two or three feet. 



The joint systems may be classed in two 

 series, of which one is younger than the other. 

 Many of the joint faces of the younger series 

 are slickensided, and some of the joints con- 

 tain veins of quartz. The older joints show 

 no slickensides and carry no veins, although 

 there is independent evidence that they are 

 planes of faulting. Their surfaces have a 

 faint but persistent undulation or mammilla- 

 tion. 



Where two fault planes of the older systems 

 intersect, each is dislocated by the movement 

 along the other, but the dislocated parts are 

 connected by a fluted surface suggestive of 

 an ogee molding. This phenomenon is sup- 

 posed to indicate simultaneous (or alterna- 

 ting) movement on the intersecting planes 

 while the rock was within (or at the border 

 of) the zone of flowage. Similar movement 

 in the zone of fracture produces splintering 

 or crushing at the intersections. 



The joint systems are interpreted as the re- 



