May 17, 1895.] 



SCIENCE. 



559 



bj' retrogression ; the main waterways di- 

 verge from the main valleys, and cut 

 through the ranges and athwart the val- 

 leys ; and the primary and secondary di- 

 vides do not coincide with the mountain 

 ranges, but traverse the valleys in a singu- 

 larly erratic manner. By reason of the 

 combination of epeirogenic and meteorologic 

 conditions, the region aftbrds a remarkaljle 

 example of the reti'ogression of streams and 

 of the development of unusual topograpliic 

 forms thereby. 



Whitman* Cross. ' The Geology of the 

 Cripple Creek Gold Mining District, Col- 

 orado." This important new gold district 

 lies on a granite plateau, some ten or twelve 

 miles southwest of Pike's Peak, at an eleva- 

 tion of 9,000 to nearly 11,000 feet. There 

 is at this point a small volcanic vent, to be 

 regarded as an outlier of an extensive vol- 

 canic region to the westward, lying between 

 South Park and the Ai-kansas Kiver. 



While the area of the Cripple Creek vol- 

 cano is small, there has been a very com- 

 plete cycle of events at this center. Explo- 

 sive eruptions in the earlier periods Ijuilt 

 up a cone of fine tufl" and breccia, through 

 which numerous eruptions in narrow fis- 

 sures and irregular channels took place in 

 later times. Erosion has now removed a 

 large part of the ejected material, though 

 not clearly disclosing tlie volcanic neck. 



The igneus products of the volcano are 

 andesites of several kinds, phonolite, pho- 

 nolitic trachj'te, nepheline-syenite, sj-enite- 

 porphyry, and several dense varieties of 

 basalt. Phonolite is the specially charac- 

 teristic rock of the center, and in dike form 

 in granite occurs for several miles about it. 



Fumarole and solfataric emanations of 

 chlorine, fluorine and sulphurous gasses un- 

 doubtedly characterized certain periods of 

 the volcano, followed by hot waters con- 

 taining the same agents in solution. By 

 these processes the rocks of the district have 

 been very extensively decomposed. The 



ore depo.sits are ver\- intimatelj- connected 

 with the volcanic center. 



This communication presented the general 

 geological results of a detailed study of the 

 district made last fall. An examination of 

 the ore deposits was made at the same time 

 by Prof. R. A. F. Penro.se, Jr., and the two 

 reports, with a geological map, will be issued 

 by the U. S. Geological Sui-vey during the 

 coming summer. 



W. H. Weed. ' Tlie Shonkin sag, an 

 abandoned channel of the ^lissouri river.' 

 The Shonkin sag is a peculiar topographic 

 feature of the countrj- south of the big bend 

 of the Missouri River in central ilontana. 

 It is an abandoned river channel which was 

 formed bj' the waters of the Missouri River 

 flowing around the margin of an extension 

 of the great Canadian ice sheet (the Lauren- 

 tide glacier). The sag consists of a wind- 

 ing vallej' fi"om a quarter of a mile to two 

 miles wide with rocky l)lufl' walls, and holds 

 a succession of lakes, several of them with- 

 out outlet. The continuity of the channel 

 is interrupted Ijy modern stream valleys 

 cutting it transversely, but their later origin 

 is clearly apparent, and even the settlers of 

 the region recognize the fact that the .sag is 

 an old water waj'. It begins near the mouth 

 of Highwood Creek, east of the Great Falls 

 of the Missouri River, and extends in a 

 general easterly direction over 100 miles to 

 the mouth of Judith River. Throughout 

 its course the northern wall marks the limit 

 of the glacial moraine. Glacial drift is 

 found in a few places a short distance south 

 of the channel, but in small quantity. In 

 general the sag defines the moraine front. 

 It is, therefore, Ijelieved that the ice sheet 

 ponding the waters of the Jlissouri near the 

 mouth of Sun River deflected the stream, 

 which at that time flowed northward, and 

 caused it to flow about the margin of the 

 ice. Upon the recession of the glacier the 

 river abandoned this temporary channel for 

 the old vallev to the northward, which was 



