100 RE VIEWS 



Physiography and Geology of the Siskiyou rajige. By F. M 

 Anderson. 



The Siskiyou range forms the central portion of the mountain sys- 

 tem often referred to as the Klamath mountains. It follows approxi- 

 mately the boundary line between Oregon and California, and, 

 branching toward the west, divides the drainage of the region in three 

 directions, on the north toward Rogue river, on the south toward the 

 Klamath river, and on the west toward Smith river. The topography 

 of the range, together with its main geological features, suggests a sys- 

 tem of transverse folds, one of which is older than the other. The areal 

 geology of the range consists of an alternate succession of igneous and 

 sedimentary, or metamorphic, belts crossing the major axis of the 

 range nearly at right angles. The axis of the range is interpreted as 

 being an expression of, or representing, an earlier topography, and the 

 geological zones as representing a later system of folding. 



There is a varietyof both igneous and sedimentary, or metamorphic, 

 rocks. The igneous rocks range from ultra-basic to acid types, and 

 are mainly deep-seated. Three belts of igneous rocks cross the range. 

 The sedimentary rocks are for the most part Palaeozoic in age, though 

 Triassic rocks may also occur. They include radiolarian cherts, sili- 

 ceous and calcareous slates, limestones, and crystalline schists. The char- 

 acter of the metamorphic rocks varies in accordance with the character 

 of the original sediments and with the character of the plutonic which 

 has effected their metaraorphism. 



Magmatic differentiation in the plutonic rocks is extreme, the gran- 

 itic rocks passing by gradations into diorites, gabbros, and other 

 pyroxene-bearing rocks. 



The Genesis of Ore Deposits in Boulder County, Col. By Rufus M. 

 Bagg, Jr., Brockton, Mass. 

 The geology of Boulder county was outlined. The formation of 

 veins was considered. A description was given of the occurrence of fis- 

 sure veins along irregular fracture zones which, after faulting, have been 

 secondarily filled with solutions and sublimation products, chiefly the 

 tellurides of gold, mercury, and the sulphide of iron, marcasite. The 

 genesis of these rich ore pockets was discussed. A description of some 

 of the principal mines was given. A summary of field observations 

 when examining: mines in the district was a:iven. 



