1898.] PECKHAM — THE GENESIS OF ]iITUMENS. 129 



traps of that region, thin veins occur of anthracitic material, which 

 alone remains to,represent the organic constituents of the altered 

 sediments. Continuing our course southwestward the same changed 

 condition is observed in the crystalline schists of Manhattan 

 Island, and across the Hudson through northern New Jersey. In- 

 trusions of trap, too, are frequent through all this region and the 

 sole representative of the organic constituents of the sediments is 

 anthracitic residues. 



On the western slope of the Catskills, through eastern New York, 

 the crystalline rocks which exist at varying depths below the sur- 

 face are overlaid with sediments which are frequently imperfectly 

 metamorphosed, and as one moves westward into central New York 

 and northeastern Pennsylvania, while the coal beneath the surface is 

 anthracite and the residues before mentioned that fill cavities in the 

 limestone are anthracitic, still the surface rocks show less and less 

 signs of alteration. As the summit of the Alleghanies is reached and 

 passed, the coal beds fade by insensible stages from anthracite into 

 unaltered splint and cannel coals. The beds of slate also become 

 beds of pyroschists, and the formations generally assume the as- 

 pect of unaltered sediments. On the western slope of the Alle- 

 ghanies the surface descends much less abruptly than it ascends on 

 the eastern slope. The dip of the formations is much greater than 

 that of the surface, consequently the outcropping edges of newer 

 formations are repeatedly encountered, until in western Pennsylva- 

 nia and New York metamorphism has ceased to be a problem in 

 surface geology. These surface rocks are, however, geologically 

 all below the coal, which in eastern Pennsylvania is metamor- 

 phosed into anthracite- There is no arbitrary line that separates 

 the unaltered from the altered strata. The successive formations have 

 thinned out, and in general they continue to become thinner as we go 

 southwest ; but there is no anthracite between the crest of the Al- 

 leghanies and the mountains of Arkansas. Throughout the Mis- 

 sissippi valley, as we pass to the west, these formations outcrop and 

 overlie each other precisely like the shingles on a roof, with the 

 pitch reversed. 



In the Bradford oil field, in McKean county, Pa., the drill pene- 

 trates a bed of porous sandstone that lies enclosed in impervious 

 unaltered strata. It contains a few shells and fish bones, but no 

 other fossils. Like the surface rocks it lies sloping toward the 

 southwest, the lower portion submerged in salt water, the middle 



PROC. AMER. PHILOS. SOC. XXXTII. 157. I. PRINTED JUNE 15. 1898. 



