158 SMITHSONIAN MISCELLANEOUS COLLECTIONS \'0L. 75 



19, where the ])re-Cambrian with the superjacent Lower and Middle 

 Cambrian is thrust over the Devonian of Fossil Mountain. Tilted up 

 at a steep angle beneath the Devonian (fig. 20) are the Ordovician 

 ( Sarbach ) . Ozarkian (Mons). Upper Cambrian (Sabine, Lyell, Bos- 

 worth and Arctomys), and Middle Cambrian (Eldon), and finally 

 the Eldon is thrust over on the upturned strata of Devonian age. 



Ulrich, in discussing the overthrust troughs within the Appalachian 

 Geosyncline, says, " The evidence on which this 1)elief in distinct 

 troughs is based is threefold in character, faunal, lithologic, and 

 structural. The first is shown by differences in fossil contents. * * * 

 The second is expressed, first by peculiarities in the succession of the 

 various types of sediments, and second, the degree of metamorphism 

 to which the deposits have been subjected. The third component of 

 the evidence is the physical proof of excessive folding and over- 

 thrusting shown by the structure of the various rock masses." ' 



The following descriptions of the Cordilleran troughs include the 

 criteria on which they have been distinguished and named. 



BOV/ TROUGH 

 Plate 25 



The area considered to have been included in the Bow Trough now 

 has a width, on a line from the Devils Gap at Ghost River to Mt. 

 Stephen (/ to P on map, pi. 25), of about 40 miles (64.4 km.). The 

 trough was narrow and confined to the eastern portion of the Cor- 

 dilleran Geosyncline when the waters of Lower Cambrian time first 

 flooded it and began to deposit siliceous silts and fine sands derived 

 from the pre-Cambrian Beltian rocks on the gently sloping eastern 

 shores of the seaway and brought into it by tributary streams. As 

 this early trough deepened, more than 2,000 feet (609.6 m.) of sands 

 and silts accumulated before the waters overlapped the margins of 

 the trough on the east and west and deposited sand and arenaceous 

 and calcareous muds during the closing period of Lower Cambrian 

 time. This widening of the trough extended westward as far as Mt. 

 Stephen {P on map) and eastward to the line of the Ghost River 

 (/ on map) at Devils Gap. At this time the Cambrian sea penetrated 

 into the Beaverfoot Trough (/, K, M, X on map) and brought with 

 it a late Lower Cambrian ( Mesonacidae) fauna similar to that in the 

 Bow Trough (see figs. 15, 16). 



The broad Bow Trough gradually deepened, the sands and siliceous 

 silts being succeeded by calcareous sediment with interbedded sili- 



' Bull. Geol. Soc. America, Vol. 22, 191 1, pp. 442, 443. 



