362 abstracts: geology 



concrete is not affected by the passage of the current. Parts two and 

 three of the paper consist of a discussion of the possibilities of trouble 

 from stray currents occurring in practice, and precautions which should 

 be taken to prevent damage where it is Hkely to occur. B. McC. 



GEOLOGY. — Coal fields of Grand Mesa and the West Elk Mountains, 

 Colorado. Willis T. Lee. Bulletin U. S. Geological Survey No. 

 510. Pp. 237 with maps, sections, and illustrations. 1912. 



The coal fields of Grand Mesa and the West Elk Mountains lie in 

 central western Colorado. Grand Mesa is a large lava-covered table- 

 land rising about 10,000 feet above sea-level and 5000 feet above the 

 surrounding country. The coal beds underlie the mesa and outcrop in 

 its sides. The West Elk Mountains consist principally of igneous rock 

 both intrusive and extrusive. The most conspicuous peaks are formed 

 by the laccoliths exposed by erosion of the sedimentary rocks that once 

 covered them. 



The sedimentary rocks described are principally of Cretaceous and 

 Tertiary age. The Dakota sandstone rests unconformably on the Gun- 

 nison formation which contains fossils that correlate it with the Mor- 

 rison, and is overlain by the Mancos shale which is about 3000 feet 

 thick and which is of Colorado age below and Montana above. This is 

 followed by the Mesaverde formation nearly 3000 feet thick which is 

 divisible into a basal sandstone (Rollins); a coal-bearing member of 

 marine and brackish water origin (Bowie) ; a coal-bearing member of 

 fresh-water origin (Paonia) ; and a thick series of undifferentiated rocks. 

 The non-marine beds contain fossil plants that seem to show that they 

 are younger than Mesaverde. The Paonia shale is separated from the 

 Bowie by an unconformity. (In the official publication these shales 

 are given the rank of members of a formation. The writer personally 

 dissents from a decision whereby rocks both above and below an 

 unconformity are placed in a single formation and believes that the 

 Rollins, the Bowie, and the Paonia, which ought properly to include the 

 undifferentiated beds overlying it, should be raised to the rank of 

 formations.) 



The Tertiary beds lie unconformably upon the Mesaverde and con- 

 sist of the Ohio Creek conglomerate, the Wasatch, and the Green River 

 formations. The Wasatch of the eastern part of the area described was 

 formerly known as the Ruby formation, but this name has been aban- 

 doned inasmuch as the Ruby seems to be a part of the Wasatch. 



Coal beds of little economic value occur at the base of the Mancos 

 shale. High grade, bituminous coal is found in the Bowie shale, and a 



