436 ORIGIN OF THE APPALACHIAN COAL STRATA, 



ginia, where a few bold axes of elevation have thrown the coal 

 rocks into a series of long, parallel, and nearly united troughs. 

 Considering all of these outlying portions of the formation as sub- 

 ordinate and intimately connected parts of one great bituminous 

 coal-field, the southeastern boundary of which is the escarpment 

 of the Allegheny and Cumberland mountains, the dimensions of 

 the great basin will be nearly as foUows : Its length, from north- 

 east to southwest, is rather more than seven hvindred and twenty 

 mUes, and its greatest breadth about one hundred and eighty 

 miles. Upon a moderate estimate, its superficial area amounts 

 to sixty-three thousand square miles. 



There are besides this, however, several smaller basins which 

 lie to the southeast, and are entirely separated from it. These 

 consist of the detached troughs of anthracite, in eastern Pennsyl- 

 vania, and the solitary outlying basin of semi-bituminous coal in 

 Broad Top mountain, near the Juniata river. The total area of 

 the coal strata in the anthracite district, may be approximately 

 stated at about two hundred square miles, whUe the semi-bitu- 

 minous formation of Broad Top is comparatively limited. _ 



Though the deep anthracite basins abound in cmious struc- 

 tural features, and contain thick seams of coal, they chiefly inter- 

 est us at present, by the geographical position which they occupy. 

 More than forty miles distant from the general denuded margin 

 of the main or western coal-field, they nevertheless present, in 

 the character of then- strata, and of the rocks upon which they 

 repose, unequivocal evidence that they and the bituminous basins 

 were once united. In this identification, we are presented mth 

 an amazing picture of the former extent of our carboniferous de- 

 posits. The existing southeastern limit of the coal, in these insu- 

 lated basins, lies, in Pennsylvania, only a short distance to the 

 northwest of the great Appalachian valley, and a survey of all the 

 circumstances involved in the question of the ancient physical 

 geography of the formation, convinces me that it extended, both 

 in that State and Virginia, at least as far to the southeast as that 

 valley. To enter here into all the facts and reasonings upon 

 which this inference is founded, would lead me aside from the 

 main purpose 'of the present paper ; but I may mention, as one 



