402 NEW YORK STATE MUSEUM 



null. These prolonged escapes of gas and oil constitute most of 

 the so-called " surface indications " of petroleum. 



The system of arches and folds above named, that find their 

 chief development in mountain regions, and to which, in fact, 

 the mountains mainly owe their origin, are the results of the 

 contraction of the crust of the earth, apparently due to its cool- 

 ing. In the Appalachian region of Pennsylvania, Claypole has 

 calculated that the shortening in of the original crust has 

 amounted to 88 miles out of 153 miles, the latter having been 

 reduced to 65 miles. Heim has calculated that an original ex- 

 tent of 203 miles in the Alps has been reduced by folding and 

 crumpling to 130 miles. The arches seem to have resulted from 

 lateral pressure exerted from the side of the ocean. Their axes 

 are approximately parallel to the ocean boundary. Their slopes 

 are gentle on the southeast side and much sharper on the north- 

 west. The strongest folds, as a rule, lie farthest to the eastward. 

 Certainly they diminish in both hight and dip as they are fol- 

 lowed westward. In western Pennsylvania, for example, the 

 folds are so reduced that they do not necessarily form the up- 

 lands of the region. They can be followed only by determining 

 the elevation of some well-marked bed or stratum, as a seam of 

 coal or sheet of limestone, or some persistent bed of red or blue 

 rock, the peculiarities or the composition of which are well 

 known. By such facts, the reality of the arch is demonstrated 

 and their directions and angles of pitch can be determined. 



All the valuable accumulations of petroleum and its deriva- 

 tive, natural gas, in Pennsylvania, are confined to these flattened 

 and dying arches, the slopes of which seldom exceed two or three 

 degrees, and which generally need to be read in minutes instead 

 of degrees. No accumulations are known where the arches show 

 angles of descent of five or 10 degrees or more. It would seem 

 that the strata were cracked in the bending, thus allowing the 

 escape of all mobile substances inclosed in the porous rocks of 

 the series. 



Following the effects of the Appalachian revolution still 

 farther westward, we come to the still feebler arches and mono- 

 clines of Ohio. Numerous cases have been found during the last 

 few vears in which the elevations of even the summits of the 



