52 J. J. STEVENSON PENNSYLVANIA ANTHRACITE. 



Professor Lesley sug,Qjests, in addition, the factor of pressure and that 

 of plant variation. All of these suggestions were offered merel}^ as sug- 

 gestions, not to formulate any hypothesis, but that others may be led to 

 a careful consideration of the matter. 



Discussion of Hypotheses based on Necessity of Metamorphism. 



Rogers and Owen regard anthracite as due to metamorphism and 

 Lesley appears to favor the same theory of origin. Each of these geolo- 

 gists presents a hypothesis respecting the origin of the heat causing the 

 metamorphism. 



ANTHRACITE CAN BE FORMED BY CONTACT WITH HEATED ROCK. 



Unquestionably, heat is sufficient, under proper conditions, for the 

 conversion of bituminous coal into anthracite. The localities in which 

 that conversion has occurred are too numerous to admit of question in 

 this connection. 



Examples of Contact- alteration in New Mexico. — Newberry, Le Conte, 

 Hayden and Stevenson have referred in various publications to the small 

 Placer coal-field about 25 miles south from Santa Fe, New Mexico. 

 There the conditions are perfectly clear. The coal is bituminous near 

 Galisteo creek, but is found becoming less and less so as one ascends 

 the arroyos (dry water-courses) leading up the mountain side, until at 

 length it is an anthracite, with 13 as its ratio. The change was produced 

 by act enormous trachyte dike, which cuts all the beds along the north- 

 erly face of the mountain, but the extent of change becomes distinctly 

 less as distance from the dike increases. This little field gives a positive 

 illustration that something more than the mere contact with molten rock 

 is needed to cause metamorphism, for in an arroyo leading up from 

 Galisteo creek a narrow dike of basalt has cut two thin beds of coal, 

 which appear to be unchanged even at the contact, for pieces taken 

 thence burned with abundant flame.* Many canyons in the Trinidad 

 coal-field of northern New Mexico show coal converted into cokef by 

 intruded sheets of basalt. No anthracite has been observed in this field, 

 but the coke is usually denser than that obtained in Belgian ovens. 

 Fragments of graphite have been reported from one exposure on the 

 Upper Canadian river. 



Example of Contact-alteration in Colorado. — T'he Elk mountains of central 

 Colorado hold an area of anthracite which was described by W. H, 

 Holmes J: and Dr A. C. Peale§ in 1876, the exposures having been ex- 



*Steven,son: U. S. Geograph. Surveys west of the 100th Meridian, vol. ill, 1881, suppl., p. 832. 

 t Log. cit., pp. 204, 208, 21C., 208. 



X Holmes : Ann. Rep. of the U. S. Geol. and Geog. Survey of the Territories for 1874, p. 67, 

 g Peale in same, pp. 98, 99, 139, 176. 



