DIASTROPHISM AND THE FORMATIVE PROCESSES 323 



the origin of the Cincinnati geanticline at this time as inconclusive, 

 and, instead, places its inception between the Niagaran and the 

 Middle Devonian, while it is recognized that the geanticline owes 

 its present proportions to later upwarping which occurred probably 

 in post-Mississippian times. ^ Minor oscillations and tilting, accord- 

 ing to Ulrich, occurred in the region of the Cincinnati and Nashville 

 domes much earlier (during the Ordovician period) but these move- 

 ments were not of the more pronounced type.^ 



In North America the principal orogenic disturbances of the 

 early Paleozoic thus occurred near the close of the Ordovician and 

 were located near the Atlantic border where thick sediments had 

 been accumulating. Europe seems to have behaved similarly. 

 Throughout much of continental Europe the relatively quiescent 

 conditions of the Ordovician continued with little interruption into 

 the Silurian. But in the British Isles, where the Cambro-Ordovician 

 strata are very thick, there were pronounced orogenic movements 

 which were approximately contemporaneous with the Taconic revo- 

 lution of North America. Over large areas the Lower Silurian rocks 

 were upheaved, more or less contorted, and in many places suffered 

 a great amount of denudation before the deposition of the Upper 

 Silurian strata began.^ A large portion of the British Isles became 

 land, and Jukes-Browne believes that this continental land included 

 also a large part of the North Sea and nearly the whole of Norway.'' 

 The results of this disturbance are conspicuous in the original tract 

 of Siluria (western England and the adjacent portion of Wales) 

 where a decided unconformity separates the Ordovician from the 

 Silurian. In some places the latter extends across the trunkated 

 edges of the former, group after group, till they rest directly upon 

 the Cambrian beds. Ramsay's diagram of the section between 

 Church Stretton and Chirbury in Shropshire shows nearly hori- 

 zontal Llandovery (basal Upper Silurian) beds cutting across nearly 



' A. F. Foerste, "The Ordovician-Silurian Contact in the Ripley Island Area of 

 Southern Indiana, with Notes on the Age of the Cincinnati Geanticline," Am. Jour. 

 Sci., 4th Ser., XVIII (1904), 321-42. 



^ E. O. Ulrich, Bull. Geol. Soc. Am., XXII (191 1), 416-19. 



3 A. C. Ramsay, The Physical Geology and Geography of Great Britain, 6th ed 

 (1894), p. 74. 



4 A. J. Jukes-Browne, The Building of the British Isles (1911), pp. 94) 97- 



