6 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



on tectonic maps which attempt to show all structures evolved through 

 three eras. I have found it desirable to think of certain basins in a re- 

 stricted time as well as restricted geographic aspect, and to prepare ac- 

 cordingly the tectonic maps that accompany this book. 



Geosyncline 

 According to Kay ( 1951 ) : 



The term geosyncline should be restricted to a surface of regional extent sub- 

 siding through a long time while contained sedimentary and volcanic rocks are 

 accumulating; great thickness of these rocks is almost invariably the evidence 

 of subsidence, but not a necessary requisite. Geosynclines are prevalently 

 linear, but non-linear depressions can have properties that are essentially 

 geosynclinal. 



Classifications of geosynclines are discussed by Kay, who takes the 

 position that all basins having a thick sequence of sediments are one kind 

 or another of geosyncline. However, only two geosynclinal terms will be 

 used in this text, namely, miogeosyncline and eugeosyncline, which are 

 the large linear basins along the margins of North America. 



Miogeosyncline 



A miogeosyncline is part of the great linear border geosyncline. It lies 

 between the shelf regions of the stable interior of the continent and the 

 outer part of the geosyncline. Its sediments are dominantly sandstone, 

 shale, chert, limestone, and dolomite, almost free of volcanic rock. 



Eugeosyncline 



An eugeosyncline is the outer part of the border geosyncline and is 

 characterized by an abundance of volcanic rock. In addition there is much 

 graywacke, arkose, dark shale, and chert. The strata are generally altered 

 by low-grade metamorphism. 



Landmass 



Landmass has no specific structural meaning unless used locally as in 

 the Ancestral Rockies of New Mexico, for instance, where an ancient 

 range is referred to as the Pedernal Landmass. The term usually con- 



notes a land area whose elevation, climate, and life are the special object 

 of study through the intermediary of the sediments derived from it, or 

 whose changing shore fines form the basis of some paleogeographic study. 

 The term does not usually imply size, relief, or origin, and no specific 

 attributes will be affixed to it in this book. 



Highland 



In Colorado, two principal uplifts dominated the structural evolution 

 of the area in late Paleozoic time, and they have been referred to by 

 most writers as highlands. They are about 50 miles wide and 200 miles 

 long and structurally were rather abrupt, asymmetrical anticlines which 

 may have been faulted in part along their steep flanks. Except in appli- 

 cation to the Colorado uplifts, the term is used very broadly in the United 

 States, and no one to my knowledge has attempted to define it; nor is it 

 necessary here to do so. It does not seem consistent, however, to say a 

 certain highland was a Zotu-lying area, but the statement may appro- 

 priately be made of a landmass. 



Ridge 



The buried Nemaha uplift of Oklahoma, Kansas, and Nebraska is gen- 

 erally spoken of as the Nemaha Mountains, but the term Nemaha ridge 

 has also been used, with the implication that ridge has a certain structural 

 significance. The use is almost unique to this area, as far as I know. A 

 ridge, topographically, is generally less than 5 miles long, and its use 

 structurally for the Nemaha Mountains, 200 miles long, is somewhat mis- 

 leading. It is not necessary to use the term in the present review. 



The term is used in oceanography to depict very large linear relief 

 features on the ocean floor, such as the Mid-Atlantic Ridge (also Rise) 

 or the Beata Ridge in the Caribbean Sea. 



Geanticline 



The term geanticline was proposed by Dana in 1873 ( Schuchert, 1923 ) 

 for "the upward bendings in the oscillations of the earth's crust — the 

 geanticlinal waves or anticlinoria." According to Schuchert, Dana's typi- 

 cal example was the Cincinnati arch, though later on, Dana also included 



