1877.J 5«J^ [Ahhburner. 



Notes to explain the above Section. 



The Section, althougli a continuous one of Carboniferous, Devonian and 

 Silurian strata, is subdivided into groups wliicli represent the long since 

 recognized and generally accepted formations of the State Geological Re- 

 ports of New Jersey, Pennsylvania and New York. 



The original numbering of these formations from I to XIII is the oldest 

 attempt at a subdivision of the Palaeozoic system, having been made in 

 1836. The numbers were used as a provisional nomenclature in publish- 

 ing the Annual Reports of the First Survey of Pennsylvania, They have 

 been used more or less in all subsequent surveys by geologists. They are 

 retained in this section as convenient symbols for ready reference, although 

 they have lost much of their lithological and still more of their palajonto- 

 logical value. 



The geographical nomenclature of the New York Final Reports of 1843 

 and 1844, as well as the more poetical names afterwards adopted by Profes- 

 sor Rogers for his Geology of Pennsylvania, published in 1858, are given 

 with the numbers, although the formations to which they have been as- 

 signed do not in all cases exactly correspond as to their upper and lower 

 limits. 



Some new names will also be noticed, chiefly near the top of the section. 

 Potisville Conglomerate, Mauch G/nink Red Shale and Pocono Sandstone 

 are geographical synonyms for XII, XI and X, — or for Serai, Umbral and 

 Vespertine, — proposed by the present State Geologist, to fill the gap at 

 present existing in the geographical nomenclature of the Palaeozoic rocks 

 between the Catskill Formation and the Alleghany River Coal Measures. 

 As IX and X together make up the mass of the Catskill Mountains, jei 

 have always been regarded as separate formations, and as there is a 

 geological objection to distinguishing them as Lower and Upper Catskill, 

 the fact that X constitutes the mass of the Pocono Mountains in Penn- 

 sylvania, has been taken advantage of to provide it with an analogous 

 geographical and euphonious name. 



Mauch Chunk and Pottsville, where XI and XII have their maximum 

 development, are important and well-known places, and there seems to be 

 no valid objection to applying these names to those formations. 



Much lower down in the Section a subdivision of the Lower Helderberg 

 group, a limestone formation of unusual thickness, is named from the 

 borough of Lewistown, one of the most important places on the Juniata 

 River, where this limestone attains its maximum of size. A special name 

 for the limestone is called for, because it cannot yet be certainly identified 

 with any special one member of the Lower Helderberg Series. 



As to the thicknesses assigned to the divisions and subdivisions of the 

 Section, it must be understood that they were obtained partly by direct 



PROC. AMER. PHTLOS. SOC. XVI. 99. 30 



