d s "J II. S. WILLIAMS — THE CUBOIDES ZONE AND ITS PAUNA. 



geologic period over the whole region may be regarded as recording approxi- 

 mately contemporaneous events, and their faunas as living at the same time. 

 In the second case, evidences may be gathered to correlate the two series 

 within broad limits: but when a wide ocean separates the two sections, corre- 

 lation of the subdivisions of the grand systems of geology is in a high de- 

 gree hypothetical, and though in text-hooks and systematic works it may be 

 pardonable, for practical purposes it is of very little value. 



But the geologist, and particularly the paleontologist is constantly called 

 upon to compare the geologic history of different continents; and while it 

 has become apparent that each continent must have its own standard scale 

 of geologic uidts, it is also of great importance to find, if possible, some 

 points in the several standards where precise correlation is practicable. 



The following paper is an attempt to establish such a point of contem- 

 poraneity in the standard geologic time scales of Europe and America for 

 the upper Paleozoic. 



In the preparation of this paper the facts regarding the rocks and faunas 

 of New York are derived from personal examination and from notes and 

 collections made for the United States Geological Survey by Mr. Ira Sayles 

 under my direction. For the facts regarding the foreign Devonian I am 

 indebted, for England, chiefly to the works of Murchison, Phillips, David- 

 son, Etheridge, Sowerby, T. M. Hall. \Y. A. E. Ussher, G. F. Whidbourne, 

 and to personal examination of the collection- of the last three gentlemen 

 and those in the Jerniyn Street and South Kensington Museums, and of 

 the sections of North and South Devonshire; for continental Europe and 

 Asia to the works of Kayscr, Barrois, Gosselet, Dewalque, Mourlon, Oeh- 

 lert, C. F. and F. A. Etoemer, Geinitz, Schnur, Grunewaldt, Keyserling, — 

 Murchison, Verneuil and Keyserling, — Tschernechew, Venukoff, von Eticht- 

 hoi'eii ; but especially to the writings of Emanuel ECayser, whose critical 

 Studies of the Devonian fossils of both EDurope and Asia are invaluable. 



THE Pimm iri.i .- OF CORRELATION. 



In discussing geologic formations of different regions of the earth, the 

 logisl requires a method of classification of terranes and a system of notation. 



The classifications in use are those based I 1 I upon the mineral constitu- 

 tion or structure of the rocks, (2) on their stratigraphic sequence, and for 

 sedimentary rocks (3) on their fossil contents. 



For the normal sedimentary rocks I which alone are discussed in tin- paper) 



correlation of t wo separate terra ne-. ha- 1 n attempted, first, by comparison 



of the constitution of the rocks themselves. This method, except for lim- 

 ited areas, i- unsatisfactory, other evidence having conclusively Bhown that 



