92 Reviews — United States Bulletins. 



Parts i and ii of this report describe the chemical and physical 

 properties of Portland cement materials, but the greater part of 

 the book deals with the cement resources of individual States. This 

 involves a description of the main occurrences of limestone, chalk, 

 and shell-marl in the United States. It is shown that the value 

 of the raw materials depends on their location with respect to fuel, 

 transportation routes and markets, and, accordingly, these points 

 are considered in each case. The distribution of the materials is 

 illustrated by geological maps wherever possible. Short bibliographies 

 are given under the several States, and there is a good general 

 bibliography, which has been compiled by F. Burchard. 



Bulletin 527. Ore Deposits of the Helena Mining Tlegion, 

 Montana. By Adolph Knopf, pp. 143, with 7 plates (including- 

 2 maps) and 4 figures in the text. 1913. — The detailed description 

 of the mining districts of the Helena region, which occupies the 

 greater part of this bulletin, is preceded by a clear sketch of the 

 geology of the region as a whole. Tlie ore-deposits are silver-lead 

 and gold-silver deposits, and they belong to two distinct periods 

 of ore-deposition, following two periods of marked igneous activity 

 in the area. The older group of lodes and veins followed the 

 intrusion in late Cretaceous time of a great mass of quartz-monzonite 

 forming the northern extension of the Boulder batholith. These are 

 all tourmaline-bearing lodes, the ore consisting principally of galena 

 associated with sphalerite and pyrite. This appears to be the first 

 recorded occurrence of silver-lead ores associated with tourmaline. 

 Analyses and field evidence show that the tourmaline and the 

 sulphides travelled together: they were probably derived from the 

 quartz-monzonite magma, but the introduction of the ore took place 

 distinctly later than the intrusion of the quartz-monzonite and of its 

 associated aplites. The second period of ore -deposition followed the 

 extrusion of a group of dacites of Upper Miocene age, and of a group 

 of rhyolites, probably slightly younger than the dacites. The ores of 

 this period are found in fissiire veins, characterized by the presence of 

 cryptocrystalline quartz with a subordinate quantity of sulphides. 

 They are worked for their gold and silver contents. These are low- 

 temperature ore-deposits, while the older group, characterized by 

 tourmaline, were deposited at a relatively high temperature. 

 Numerous analyses are given of samples taken from veins of both 

 groups occurring in quartz-monzonite, and interesting comparisons 

 are made between the metasomatic processes which operated during 

 the two periods of mineralization. 



Bulletin 529. The Enrichment of Sulphide Ores. By "W. Harvey 

 Emmons, pp. 260. 1913. — The theory of enrichment of sulphide 

 ores, first put forward in 1900, has been found to have very wide 

 application, and has proved to be of considerable value in the 

 development of sulphide deposits. It is a theory which lends itself 

 especially to research in the laboratory, and though much Avork 

 has been done, there ai'e many problems which still remain unsolved. 

 This bulletin summarizes our present knowledge of the subject; it 

 supplies some new material and suggests lines for further inquiry. 

 The bearings of physical conditions on the enrichment of deposits are 



