218 C. D. WALCOTT JMIE-CAMBKIAN FOSSILIFEKOUS FORMATIONS 



the Grand Can^-on ])roper, and also of the lateral canyons which enter 

 it. As has already been mentioned, the plane of unconformity cuts 

 across the pre-Cambrian rocks from the highest beds of the Chuar 

 terrane to the base of the Unkar terrane and into the subjacent schists 

 and granites. This is illustrated by figure 5. 



Extent of unconformity. — The extent of the unconformity between the 

 Chuar terrane and the Cambrian will probably never be fully ascertained. 

 We new know that the highest beds of the Chuar terrane formed an 

 island in the Cambrian sea wdiich was more or less eroded prior to the 

 deposition of the Cambrian beds over its summit, the basal beds of the 

 Cambrian being deposited around its sides. The unconformity proves 

 that an extended orographic movement took place prior to the pre- 

 Cambrian erosion, and that the subsequent erosion must have been of 

 comparatively long duration to produce the baselevel on which the 

 Cambrian sediments were subsequently deposited. 



LLANO SESIES OF TEXAS 



In central Texas the basal sandstones of the Cambrian correspond, 

 lithologically and by means of the middle Cambrian fauna in their upper 

 beds, with the Tonto sandstones of the Grand Canyon section. They 

 rest unconformably on a series of alternating shales, sandy shales, sand- 

 stones, and limestones, which are very much like those of the pre- 

 Cambrian Grand Canyon series. Like the Grand Canyon series, the 

 Llano rocks are almost unaltered, showing little more evidence of meta- 

 morphism than the overlying Cambrian and Carboniferous strata. 



No systematic search for fossils has been made and none have been 

 found, but on account of similarity in stratigraphic relations and litho- 

 logic characters between the Llano series and the Grand Canyon series 

 the two have been correlated.* 



A VA LON TERRA NE 



The formntionsin general. — 'This terrane includes the formations between 

 the l)asal beds of the Cambrian and the Archean gneisses of Newfound- 

 land. In the summer of 1888 I made a rapid trip across the section 

 from Saint John harbor to Topsail head, Concei)tion bay, which gave 

 me a clear impression of the general character and order of succession 

 of the strata, but no details. For the latter it is necessary to refer to the 

 reports of Dr Alexander Murray. 



Dr T. S. Hunt gave the name Terranovan to the series of strata be- 

 tween the Laurentian gneisses and the fossiliferous strata of Cambrian 



*Am. Jour. Sci., vol. 28, 1884, pp. 4.31, 432. See also Professor Comstock's description in Second 

 Ann. Report Geol. Survey of Texas for 1890, pp. 5G2, 5iJ3. 



