2. 



STRUCTURAL TERMINOLOGY 



NEED OF STANDARD TERMS FOR REGIONAL STRUCTURES 



The posthumous work of Schuchert (1943) is an example of the ir- 

 regular use of names for the large structural features of the United States. 

 He speaks of the Cincinnati anticline and the Cincinnati geanticline, evi- 

 dently interchangeably, and the Nashville dome in the same sense as the 

 Cincinnati anticline. McFarlan (1943), in his book on the geology of 

 Kentucky, defines the Cincinnati arch as a major structure which includes 

 the Jessamine dome and the Nashville dome, but in several places he 

 refers to the arch as a dome. In Colorado the Ancestral Rockies are com- 

 monly called highlands and geanticlines, in New Mexico they are land- 



masses, in Texas they are uplifts and arches. The buried Nemaha 

 "Mountains" in Oklahoma and Kansas have been called a ridge. There are 

 a number of other terms for which no standard structural meaning has 

 evolved. The professional geologist may not experience any difficulty or 

 inconvenience in this loose and local application of names for the large 

 structural features of the earth's crust, but for the student it is confusing. 

 I have felt impelled to define and classify for his sake, because the book is 

 addressed to him. In so doing, however, I feel at many turns there will 

 be objections, largely on the grounds of provincial usage. 



In view of the undesirability of multiplying technical words, it seems 

 necessary to assign specific meanings to common words in their several 

 fields of usage. For instance, the word system when used in stratigraphy 

 denotes the rocks formed during a period of geologic time; when used 

 geographically it generally signifies a group of ranges with unifying char- 

 acteristics; and when used structurally it indicates a group of related 

 joints, faults, dikes, or the like. It is probably better to give a word such 

 as system several meanings rather than use a new word, or a less common 

 and, perchance, a less appropriate one. The commonest usage of a term 

 should weigh heavily in formulating a definition for it. 



MEANING AND CHOICE OF TERMS FOR THIS BOOK 



Arch and Dome 



From 1891 to 1903 Foerste spoke of the Cincinnati uplift as an anti- 

 cline, then in 1904 as a geanticline, and Schuchert continued the use of 

 these two terms apparently interchangeably. The first mention of the 

 terms arch and dome for the structure has not been located in the litera- 

 ture, but since 1900 they have been used very commonly and usually 

 synonymously. They are the terms used both provincially and nationally 

 most frequently today. McFarlan ( 1943 ) has distinguished the two in the 

 sense that the Cincinnati arch is an elongate structure and includes two 

 dome-shaped uplifts on it, the Jessamine dome and the Nashville dome, 

 separated by a sag or saddle. Tennesseans will probably not accept the 

 subordination of their Nashville dome to a division of the Cincinnati arch, 

 but the principle of the distinction of arch and dome is appealing. Since 





