THE STRUCTURAL CLASSIFICATION . 593 



bitumen. Moreover, in Mexico many crevices exist in the volcanic necks, 

 and these are sufficient to allow oil to enter from the surrounding Tertiary 

 and Cretaceous formations and thus pass up to the surface. 



Eateau mentions^* an instance at Roczk, Galicia, where a trachitic rock 

 is impregnated with petroleum. A similar report comes from Prof. 

 Arthur Lakes/^ who describes dikes of injected volcanic origin, more or 

 less saturated with petroleum, near Pagosa Springs, in Archuleta County, 

 Colorado. The petroleum here is also associated with hot sulphur water. 

 De Golyer''^ mentions probable examples of oil seepages from dike fissures 

 at Cerros de la Pez and de la Dicha, at Ebano, Mexico. The best example 

 known to the writer is from the side of Cerro de Chapapote, between 

 Tepezintla and Pierre Labrada, Mexico, where oil can be seen seeping out 

 of a conspicuous basalt plug from about 60 feet above its base. Wash- 

 burne^^ mentions the occurrence of small amounts of oil in porous basalt 

 on the Johnson Eanch, ou the North Fork of Siuslaw River, western 

 Lane County, Oregon. 



A somewhat different type of occurrence is found in granite and associ- 

 ated crystalline rocks on Copper Mountain, in northeastern Fremont 

 County, Wyoming, according to Trumbull. "^^ For many years asphalt, 

 oil tar, or "brea," as it is frequently called, was gathered for fuel from 

 points on the granite mountain. The geology has been worked out in de- 

 tail by Darton,"^^ who found that Copper Mountain is a dome over the 

 granite core of which stratified rocks were at one time present, having 

 been removed by erosion. The brea and deposits of heavy oil have accu- 

 mulated in hollows in the upper part of the mountain, and oil has been 

 encountered in shafts and tunnels high up in the granite. The oil was 

 originally accumulated in the Ember sandstone of Permian age which 

 overlay the dome, and when the faulting occurred some of it settled into 

 the crevices of the granite as low as the water level. The downward 

 stratigraphic migration is supposed to have been as much as 2,000 feet, 

 but the direction of migration is described as probably lateral during 

 tilting of the rocks to form the dome. 



CLASS VIII—CREVICES OF SEDIMENTARY ROCKS 



The Florence field of Colorado is shown by Washburne^^ to be due to 



^* M. A. Rateau : Annates des Mines, 8th ser., vol. 11, p. 152. 



■^5 F. H. Oliphant : Mineral resources of the United States for 1910. U. S. Geol. Sur- 

 vey (1902), p. 561. 



78 E. De Golyer : Econ. Geol., vol. 10, 1915, p. 655. 



'7 C. W. Washhurne : U. S. Geol. Survey Bull. 590. 



■78 L. W. Trumbull : Bull. No. 1, Scientific Series of the State of Wyoming, Geologist's 

 office, 1916, pp. 5-16. 



'8 N. H. Darton : Prof. I'aper 51, U. S. Geol. Survey. 



80 C. W. Washhurne : Bull. 381, U. S. Geol. Survey, 1910, pp. 521-523. 



