AMERICAN GEOLOGY DECADE OF 1870-1879. 583 



Iii L881 and 1883 Professor White studied the fossils of the middle 

 belt of counties on the Delaware, Susquehanna, and the upper Juniata 

 rivers. Professor Stevenson did the same for the Maryland border, 

 while E. W. Claypole was commissioned to prepare a special report 

 on all forms discoverable in the district of the lower Juniata. 



The main energies of the survey were, as above noted, devoted to 

 economic problems, with particular reference to the extent both geo- 

 graphic and geologic of the coal and petroleum formations. A great 

 amount of good was undoubtedly accomplished, perhaps as much as 

 one has a right to expect. Work in later years has, however, showed 

 that the fundamental defect of the survey was lack of accurate topo- 

 graphical maps. This seems the more remarkable in view of the fact 

 that the same defect became obvious during the progress of the Rogers 

 survey, and further, in that Lesley was, himself, a topographer. 



To quote Lesley's own words (manuscript), the facts of dominant 

 importance which became impressed upon the Pennsylvania survey 

 were as follows: 



(1) The Paleozoic formations reach their maximum thickness in this State; and 

 consequently admit of a greater differentiation than elsewhere into special groups of 

 beds. 



(2) The middle region of the State is magnificently plicated and eroded, exposing 

 innumerable outcrops, connected in zigzags, and of immense length. 



(3) No unconformable later deposits cover and conceal these outcrops, so that there 

 is an unexampled opportunity for the study of variable thickness and changes of 

 type. 



(4) The topographical features are so dependent upon the lithologv and structure 

 that any geological survey of the region must be virtually a topographical survey. 



(5) The geological areas are of great size and so clearly defined, and so distinct in 

 character, that they naturally claimed and received each one a survey of its own. 

 These areas are: 1, The Bituminous Coal field of the west and north; 2, the Anthra- 

 cite Coal fields in the east; 3, the middle belt of Devonian and Silurian formations; 

 4, the Mesozoic belt of the south and east; 5, the South Mountain Azoic; 6, the 

 Philadelphia belt of Azoic rock; and 7, the region of Glacial Drift. 



(6) The natural section of the Bituminous Coal Measures, down the Monongahela 

 and up the Allegheny rivers, relieved the study of that part of the Paleozoic system 

 of all ambiguity. 



(7) The great amount of mining done in the anthracite fields made that part of 

 the survey peculiarly exact and correct. 



(8) The great size and number of the brown hematite mines furnished unusual 

 opportunities for the study of that kind of mineral. 



(9) The great size and number of limestone quarries, exploited for the manufacture 

 of iron and for fertilizing farms, opened to view every part of the great Siluro- 

 Cambrian formation, the whole of the Lower Helderberg, all the Devonian, and most 

 of the Carboniferous limestone beds. 



(10) On the other hand, Pennsylvania is singularly destitute of workable veins of 

 the precious metals. Its poverty in gold, silver, copper, and lead is extreme. It has 

 but one important zinc deposit; and but one nickel mine." In fact its Azoic regions 

 as a whole are barren country, containing but a few small magnetic iron ore beds, in 



« Not worked since 1891. 



