BAKED CLAYS AND NATURAL SLAGS 409 



in thickness, but occasionally reaching a thickness of eight or ten 

 feet. The beds have been sculptured by erosion into typical "Bad 

 Lands" forms, giving the coal beds a large area of outcrop, and thus 

 greatly facilitating their combustion. 



The ignition of the coal has in certain cases taken place through 

 human agencies, as in the case of a bed now burning (summer of 

 1903) six miles west of Gillette, Wyo., which was set on fire in 1902 

 by laborers at work on the railroad; in most cases, however, we must 

 attribute their ignition to spontaneous combustion, or, at any rate, 

 to agencies other than human. At the locality above referred to, 

 the coal outcrops along the sides of a deep gulch, and its burning is 

 accompanied by the emission of much heat and of considerable vol- 

 umes of sulphurous gases. 



The unburned strata are typically of a gray or buff color, but upon 

 the burning-out of an underlying coal seam they assume most gaudy 

 hues of bright yellow, pink, or deep brick- red; the stratum beneath 

 the coal is usually but little affected. Frequently a red layer may 

 be traced from butte to butte, in each case underlain by the ash of 

 the burned-out coal seam. With the change in color go incipient 

 fusion and an increase in the coherence and resistance of the strata. 

 This increased resistance has had a marked influence upon the topog- 

 raphy, and it is common to find buttes capped with a layer of this 

 baked material which has served to retard the progress of normal 

 erosion. 



Rocks resulting from the baking and fusion above described have 

 been referred to by German geologists under the names Porzellanit 

 and Porzellanjaspis 1 and have been observed by them in a number 

 of the European coal fields. Similar materials have been observed 

 by the writer in the Coal Measures near La Salle, 111. The principal 

 varieties of altered material observed in the Wyoming region are 

 described below. 



1. By far the largest part of the metamorphosed beds consists of 

 buff, brick-red, or indian-red argillaceous material. Much of this 

 is fissile along bedding-planes, and incloses lamellibranch shells 

 and shows impressions of leaves in great perfection; other portions 

 are more massive and show a somewhat conchoidal fracture; still 



1 Zirkel, Lehrbuch der Petrographie, Vol. Ill, p. 775. 



