proceedings: geological society 141 



The most interesting rare minerals of the dikes are the niobates, 

 tantalates, and titano-niobates (colmnbite, fergusonite, euxenite, 

 blomstrandine, etc.) and the siHcates containing rare earths (thorite, 

 orthite, gadoKnite, hellandite, thortveitite, etc.). The following are 

 some of the rare elements present in these minerals: yttrium, erbium, 

 cerium, scandium, thorium, uranium, tungsten, niobium, tantalum. 

 The cordierite-bearing pegmatites occur in small veins in gneiss, and 

 contain chiefly oligoclase (sunstone), cordierite, and quartz, with 

 small amounts of magnetite, apatite, etc. The epidote-bearing peg- 

 matites occur as veins in metamorphic rocks (altered quartz-porphy- 

 ries). They contain microcline and quartz as chief minerals, with con- 

 siderable quantities of epidote, apatite, and hematite, and some biotite, 

 muscovite, and titanite. 



Discussion. E. S. Bastin spoke of the resemblance in structure 

 (rather than in mineralogy) which the Norwegian pegmatites show to 

 those of the United States, especially as regards the graphic inter- 

 growths. He thought that it was difficult to consider these inter- 

 growths as eutectics, because of the variable ratios in which the minerals 

 occur in them. He inquired as to Andersen's views on this matter. 

 Andersen said that he had not yet studied them thoroughly from this 

 standpomt, but he thought that simultaneous crystallization had been 

 a factor, and showed by a diagram that in a three-component system 

 the boundary curves might follow such a course as to give varying 

 proportions of two components which are simultaneously crystallizing 

 out. 



E. T. Hancock, The history of a portion of Yampa River, Colorado, 

 and its possible hearing on that of Green River. The relation between 

 the main Uinta uplift and two minor uplifts known as Junction Moun- 

 tain and Juniper Mountain was first pointed out, also the relation in 

 origin of these uplifts to the Axial Basin anticline, which C. A. White 

 called "the inceptive portion of the Uinta fold." The views of some of 

 the earlier geologists regarding the origm of Green and Yampa Rivers 

 were briefly cited. The author of the paper then gave the evidence 

 to show that at one time both the Juniper and Junction Mountain 

 uplifts were covered by the overlapping Browns Park formation, 

 and that Yampa River, instead of being antecedent, as C. A. White 

 and others supposed, is superimposed, and that the entrenched me- 

 anders east of tbe Juniper Mountain uplift are due to the fact that 

 for a time the uplifted Paleozoic rocks of Juniper Mountain acted as a 

 barrier. After the river succeeded in cutting its channels down through 

 the hard rocks it then deepened its channels throughout that part 

 of its course where it had previously established its meanders. (To 

 be published in complete form in U. S. Geological Survey Professional 

 Paper No. 90, Chapter K.) 



Discussion. C. W. Cross spoke of the earlier work done by S. F. 

 Emmons in this region and inquired if Emmons' reasons for believing 

 that the Green River had other than an antecedent origin had the same 

 basis as those of Hancock. Hancock replied that Emmons' conclusion 



