718 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



Tlie Middle Cambrian section is one of the most complete in Europe and its 

 abundant fauna is well preserved. The succession of Ordovician beds in the Bo- 

 hemian Basin is representative of the rocks of that system for Europe, and its 

 fossils have been fully described by Barrande. He also made known the richly 

 fossiliferous Silurian limestones of this area, including with them strata which 

 later were considered by Emmanuel Kayser and others to be Devonian. 



Lower Paleozoic rocks have a wide distribution in the Baltic region and east- 

 ward through northern Russia. The most complete succession of the Silurian 

 formations in Europe occurs on the island of Gotland, where limestones pre- 

 dominate. The Silurian rocks of Sweden and their faunas were studied by An- 

 gelin in 185-4 and, on the basis of Trilobite genera, were divided into seven 

 stages. The Ordovician system in Sweden, as in many parts of the world, is in 

 part composed of shales rich in graptolites, which have been studied by numer- 

 ous paleontologists and correlated with the similar succession in Great Britain. 



The major subdivisions of the early Paleozoic rocks of eastern North America 

 were studied by Ebenezer Emmons prior to 1850 and in the following years im- 

 portant supplementary contributions were made by James Hall, J. D. Dana, 

 H. D. Rogers, William INIather, C. D. Walcott, and many others. The final re- 

 port of Rogers in 1858 on the geology of Pennsylvania adopted in part the clas- 

 sification earlier proposed in New York State and suggested the idea of the 

 Appalachian trough as a basin of deposition, with a land mass, lying to the east 

 and partly beyond the present coast, as a source of the Paleozoic sediments. The 

 Lower Paleozoic rocks in western Massachusetts and eastern New York, like 

 those in AVales, have been strongly folded, faulted, and partially metamor- 

 phosed; they rest on gneiss, so that the problem of their classification was for 

 many years involved in controversy. Emmons thought these rocks were older 

 than the Upper Cambrian Potsdam sandstone of northern New York and pro- 

 posed for them the name Ta conic system. Numerous investigations during the 

 past one hundred years resulted in an explanation of the Taeonic problem: 

 namely, that moderate uplift during the Ordovician became accentuated near 

 its close, with folding and overthrusting, and a chain of mountains extending 

 from Newfoundland southward to New Jersey was produced. This event has 

 been termed the Taeonic disturbance; in consequence of it the Silurian forma- 

 tions rest unconformably upon the beveled edges of the older rocks. The effect 

 of this disturbance diminished toward the west, where the lower Paleozoic strata 

 are relatively horizontal. 



The widespread lower Paleozoic formations in the Mississippi Valley and 

 northern Gulf States areas were under investigation by the newly organized 

 state geological surveys during the middle and late nineteenth century. Also, 

 because of the relation of these rocks to the occurrence of oil and gas, particular 

 areas have been studied in great detail during the past fifty years. 



The name Cordilleran trough has been applied to an area in the Great Basin 

 region extending from Arizona northward into Canada, which during the Paleo- 

 zoic was at times a basin of deposition for great thicknesses of marine Paleozoic 

 rocks. Numerous studies by the U. S. Geological Survey of areas containing 

 mineral deposits have yielded stratigraphic and paleontologic information con- 

 cerning Paleozoic strata and, although different names have been applied to 

 widely separated stratigraphic sequences, a satisfactory correlation of beds is 



