CAMBRIAN AND CAMBRO-SILURIAN FORMATIONS. 175 



ains, which, in a structural sense, are of equal importance with any 

 described, but they are physiographically insignificant and without min- 

 eral resources of value. 



/ 



STRATIGRAPHY. 



The Paleozoic rocks are quite well represented in this district, with 

 the exception of the extreme top of the series. The formations show- 

 ing in outcrop are, beginning with the oldest, as follows : 



Cambrian — Graysonton Formation. — A complex mass of red and green 

 shales and interbedded limestones ; the limestones are generally silic- 

 eous, showing a gradual transition from the siliceous green shale to a 

 solid limestone, the whole highly charged with iron and giving rise to 

 deep-red soils; occasionally a bed of pure blue limestone is seen, but 

 they are not common. No reliable estimate of its thickness could be 

 made, as the succession of beds was not determined and no regular 

 section could be found that would afford a reliable measure. It is gen- 

 erally much crumpled and disturbed, and has the appearance of being 

 closely folded ; the folds do not appear to sustain any definite relation 

 to the main structural features of the region, and the writer is inclined 

 to believe that they owe their present disturbed condition more to fold- 

 ing that occurred in early Paleozoic time than to that which culminated 

 the Appalachian revolution and produced the structural features now 

 most prominent. The age of this formation cannot be definitely stated, 

 as it is apparently unfossiliferous, but seems to be perfectly conform- 

 able to the Shenandoah limestone and immediately beneath it, so that 

 it is probably of Middle or Lower Caml^rian age. 



Cambro-Silurian — Shenandoah Limestone. — This is equivalent to the Val- 

 ley limestone of Rogers, and includes the lower portion of the Silurian and 

 the upper portion of the Cambrian periods. It is very much folded and 

 crushed, and this folding is of the same type as that described in the 

 preceding formation. Its thickness is about 4,000 feet, and it is generally 

 a gray dolomite, slightly cherty at some horizons, and at its extreme top 

 there is a few feet of a blue, sparingly fossiliferous limestone, which is 

 probably the only representative of the Trenton-Chazy limestones in 

 this region. The lower portion is somewhat shaly, so that the soil is 

 apt to show some fragments of a white or yellow shale. In the vicinity 

 of Radford this formation carries heavy beds of limestone conglomerate; 

 these beds or rather masses appear to be free from bedding planes, and 

 in only one place could their attitude relative to the regularly bedded 

 limestone be determined. On the bank of the river, two miles below 

 Radford, where the railroad has made a sidehill cut, the face of the cliff 

 is well exposed and shows the contact of the conglomerate and lime- 



