abstracts: geology 89 



and anticipate the discovery of minerals in which Ta 2 B was in excess of 

 Cb 2 O5. To avoid confusion the name is retained. 



The mineral here described is found in considerable quantity as an 

 original constituent of the Etta granite pegmatite dike near Keystone, 

 South Dakota. It is black, opaque, and crystallizes in the rutile group 

 of the tetragonal system. It occurs as individual crystals, the largest 

 of which are 1.8 to 2 mm. across, by 5 mm. long, and in aggregates up 

 to 16 mm. across. The analysis was made by decomposing the mineral 

 with chlorine and sulphur monochloride, and separating the chlorides 

 thus obtained by fractional sublimation. The analysis gives the for- 

 mula Fe O. (Ta, Cb) 2 5 . 6 Ti 2 with 49.1 per cent Ti 2 , 7.5 per cent 

 Fe O, 35.7 per cent Ta 2 5 , 6.4 per cent Cb 2 5 and 1.3 per cent Sn 2 

 (probably replacing Ti 2 ). R. C. W. 



GEOLOGY. — Geology and ore deposits of the Breckenridge district, Colo- 

 rado. F. L. Ransome. Prof. Paper U. S. Geological Survey No. 

 75. Pp. 184, with maps, sections and illustrations. 1911. 



The Breckenridge district is in Summit County, Colorado, 60 miles 

 west-southwest of Denver. The fundamental rocks are granites and 

 schists of pre-Cambrian age. The thick series of Paleozoic rocks present 

 in the Leadville and Tenmile districts thins to the north and west and is 

 not represented near Breckenridge, where the oldest sedimentary rocks, 

 resting directly on the pre-Cambrian, are the red sandstones and shales 

 of the " Wyoming" (Triassic?) formation. Apparently conformable 

 above them is the Dakota, overlain in turn by a thick formation of dark 

 hales which probably represent the Benton, Niobrara, and part of the 

 Montana formations of the Upper Cretaceous. In the northern part 

 of the district the Dakota rests on the pre-Cambrian. The sediments 

 are intruded by monzonitic porphyries ranging in composition from 

 siliceous quartz monzonite porphyry to hypersthene-bearing diorite 

 porphyry. The Quarternary deposits may in part be divided into glacial 

 accumulations of Pleistocene age and stream gravels of the recent epoch. 

 There were two advances and retreats of the ice. The earlier is repre- 

 sented by terrace gravels and what has been called older hillside wash; 

 the latter by moraines and low-level gravels or valley trains. 



The principal fissures strike northeast and form a conjugate system. 

 No single fissure is known to exceed 1700 feet in length and none was 

 formed to the accompaniment of important structural displacement. 

 The ore deposits may be grouped as (1) veins of the zinc-lead-silver- 

 gold series, (2) stockworks and veins of the gold-silver-lead series, (3) 



