30 Reviews — Coal-fields of Colorado. 



belonging to the fauna of the Montana group. The Mancos shale 

 corresponds to the Montana and Colorado formations of eai'lier writers, 

 Eldridge's Fox Hill division being represented by a transitional zone 

 between the Mancos shale and the overlying Rollins Sandstone. The 

 rest of the Upper Cretaceous rocks, about 2,900 feet, are represented 

 by: (1) the Rollins Sandstone, containing Ealymenites major and 

 Inoceramus ; (2) the Bowie shale, coal-bearing throughout, with marine 

 and brackish-water shells; (3) the Paonia shale, also coal-bearing 

 but characterized by abundant plant-remains and freshwater shells ; 

 (4) an undifferentiated thickness (2,000 feet) of sandstones and shales 

 with plant-remains and freshwater shells, but containing only a little 

 poor coal. The Bowie shale is absent in the central part of the 

 Grand Mesa field, and elsewhere in the area it is separated from 

 the Paonia shale by an unconformity. 



The lower part of these Cretaceous beds was classed by the 

 geologists of the Hay den Survey of 1881 as Fox Hills Sandstone, 

 the upper part as Laramie formation : the invertebrate fauna, 

 however, indicates that they may now be correlated with the upper 

 part of the Mesaverde formation of South-Western and North- Western 

 Colorado. The evidence of the plant-remains from the freshwater 

 beds above the unconformity is less conclusive, and accordingly 

 Mr. r. H. Knowlton's notes on the flora are given in detail. He 

 concludes that the Paonia shale is certainly of post-Montana age and 

 that it may prove to be Laramie or even still younger. 



The Cretaceous rocks are overlain unconformably by the Ohio 

 Creek conglomerate which Eldridge regarded as Cretaceous but 

 which is now shown to be Tertiary. Another unconformity separates 

 this from the "VVasatch and Green River formations, a series of 

 unfossiliferous sandstones and shales, probably Eocene. 



The igneous rocks of the area were described by Whitman Cross in 

 1894. They comprise great laccoliths of quartz-monzonite porphyry 

 of Tertiary age, basalt flows, possibly early Pleistocene, and a great 

 development of andesitic breccias. 



The burning of coal at the outcrop is the subject of an interesting 

 note. This is less general on northern soil-clad slopes than on the 

 dry southern faces of the hills. No evidence was found in favour of 

 the theory of spontaneous ignition, and it is thought that the burning 

 of the coal is due to forest fires. 



In many places no coal appears at the outcrop, and quite the most 

 striking point in the book is the evidence of the fossils on the nature 

 of the buried coal which a prospector may expect to find at a given 

 locality. It is pointed out that the high-grade bituminous coals are 

 associated with the marine Bowie shale, characterized by Halymenites 

 major, Inoceramus harabi7ii, I. sagimsis, Cardium speciosxim, and Ostrea 

 subtrigonalis ; on the other hand, low-grade bituminous or sub- 

 bituminous coals are found in the freshwater Paonia shale containing 

 freshwater sliells and plant remains. Seven splendid plates depict 

 the most characteristic and easily recognizable fossils from the 

 coal-bearing beds. This forms only one of many instances in wliich 

 the United States Geological Survey have demonstrated the very 

 close relation of purely scientific to economic geology. 



