STRUCTURAL TERMINOLOGY 





1. By wide-spread deformation of the earth's crust, transmitted from place 

 to place. This leads to the elevation of many and widely separated mountain 

 ranges, . . . Each revolution ... is named after one of the prominent moun- 

 tain ranges formed at the time designated, for example, Laramide and Ap- 

 palachian revolutions. 



2. By wide-spread changes in the physical geography . . . 



3. By marked and wide-spread destruction of the previously dominant, 

 prosperous, and highly specialized organic types. 



4. By marked evolution of new, dominant, organic types out of the small- 

 sized and less specialized stocks, and by the development of hordes of new 

 species. 



With revolutions reserved to close eras, Schuchert used the term dis- 

 turbance to terminate periods. Thus the crustal movements at the close of 

 the Devonian period in New England and Acadia would be called the 

 Acadian ( Schickshockian ) disturbance. 



In light of recent research, certain disturbances are known to have oc- 

 curred within periods, and three (Taconic, Acadian, Nevadan) are equal 

 or exceed in size and certainly exceed in intensity the Appalachian (as 

 orthodoxly known) and the Laramide revolutions. In the Alps, the di- 

 astrophic history is followed from the middle Carboniferous to the close 

 of the Oligocene, and it seems difficult to apply the term revolution in 

 Schuchert's sense. The great paroxysms in which the nappes were formed 

 occurred in middle Oligocene time, and to these and all other deforma- 

 tions of early Tertiary time, Argand ( 1916) applies the name Alpine cycle. 

 Thus he speaks as follows: "The regime of deformation of Asia during 

 the Alpine cycle, . . . etc." (1922). He refers to the Hercynian cycle 

 and the Caledonian cycle, apparently in the same general way as others 

 do with the words orogeny, revolution, disturbance, and phase. 



Rucher ( 1933) adapts the term revolution to his own nomenclature and 

 theory by the following: ". . . the juxtaposition of the high welt and the 

 deep sediment-filled furrow leads to the violent deformation traditionally 

 known as 'revolutions.' " 



Refore deciding what terms or classification to use in this book, a few 

 other words need to be discussed. The terms orogeny and epeirogeny, 

 according to Gilbert (1890) are processes of deformation. He defined 

 orogeny as the process of mountain building, and epeirogeny as the 

 process of continental displacement to form the large swells and basins. 



The two processes collectively he called diastrophism. Orogenic struc- 

 tures, according to Stille (1924) are visible to the eye, such is faults 

 folds, and thrusts; whereas epeirogenic structures arc so gentle thai dips 

 are scarcely noticeable, and are due to broad warping. The usage in 

 America today is fairly uniform in the respect that orogenic movement is 

 of the nature of folding, thrusting, and block faulting or rifting and for 

 the most part takes place in the geosynclinal belts. Epeirogenic move- 

 ment is vertical, of gentle nature, and affects regional parts of the trust. 

 The arches, domes, and large basins of the central stable region of the 

 continent are examples of epeirogenic movements, and the interruption of 

 cycles of erosion in the deformed geosynclinal belts by elevation is an ex- 

 ample of epeirogenic movements in the marginal and older orogenic belts. 

 It is in this sense that the terms will be used in this book. 



A point that is confusing is the interchangeable use in our literature of 

 orogeny and revolution. It would seem from Gilbert's early usage that 

 orogeny is a process, and to say Appalacliian orogeny would be to focus 

 attention on the processes of deformation in the geosyncline — to em- 

 phasize the mechanical relations. On the other hand, to say Appalachian 

 revolution would be to broaden one's vista structurallv to the events in 

 the hinterland and the foreland as well as in the geosyncline, and to in- 

 clude the climates and changes in the organic world. Current usage of the 

 term orogeny is also often synonymous simply with crustal disturbance. 

 Angular unconformities and coarse, thick, basal conglomerates are com- 

 monly the evidence of orogenies, and the orogenies are given names such 

 as the Diablan, Santa Lucian, and early Laramide. Refore deciding on 

 definite usages of the terms, it is best to consider their time and geo- 

 graphic limits. 



Phase 



The term phase has been used structurally as well as stratigraphicallv. 

 In nearly all structural uses it is a division, either spatial or time, of a 

 revolution. For instance, Argand (1922) in explaining his tectonic map of 

 Asia says, ". . . we have concluded . . . that a classification of the ele- 

 ments (shows) only the age of the principal folding . . . neglecting the 

 phases but retaining the orogenic cycles." And again. ". . . all the pli 



