450 BULLETIN 109, UNITED STATES NATIONAL MUSEUM. 



The publication of the later reports was regulated by section 2 

 of the act of July 3, 1885, which reads us follows : 



That of each report hereafter published, 3,500 copies shall be printed, which 

 shall be distributed by the boai'd of commissioners as follows: Five hundred 

 copies to the senate ; 2,000 copies to the house of representatives ; 150 copies to 

 the State geologist, out of which he shall donate copies to authors and to mem-* 

 bers of survey corps as heretofore granted ; 600 copies to the board of com- 

 missioners for distribution to public libraries, universities, and colleges in the 

 State, to parties rendering material assistance to the survey, and for exchange 

 with foreign societies and geologists ; and 50 copies thereof to the members of 

 the board; 100 copies to the secretary of internal affairs, for distribution by 

 him ; 100 copies to the governor, for distribution by him ; and 50 copies to the 

 State librarian, for distribution and exchange with other States and Territories. 



The edition of the hand atlas was limited to 1,000 copies. 



Benefits and results. — A special character has ])een impressed 

 upon the Pennsylvania survey by several facts of dominant im- 

 portance : 



1. The Paleozoic formations reach their niaxiiiuim 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 magnificiently plicated and 

 eroded, exposing innumerable outcrops, connected in zigzags, and 

 of immense length. 



3. No unconformable later deposits cover and conceal these out- 

 crops, so that there is an unexampled opportunitj^ for the study of 

 variable thicknesses and changes of type. 



4. The topographical features are so dependent upon the lithology 

 and structure that any geological survey of the region mu.st be vir- 

 tually a topographical survey. 



0. The geological areas are of great size and so clearly defined 

 and so distinct in character that they naturally claim.ed and received 

 each one a survey of its own. These areas are: 1, the bituminous 

 coal field of the west and north; 2, the anthracite 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 Movmtain 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 fur- 

 nished unusual opportunities for the study of that kind of mineral. 



