PALEOZOIC GROUP. 287 



Hickory Series, dipping south about 1 5 degrees, and underlaid "by the Texan 

 Beds, dipping in the same direction 28 degrees, are overlaid by a thin capping 

 of coarse grained, dark red, pebbly sand rock, dipping a few degrees north- 

 west. This mountain is badly faulted, and contains, therefore, many puz- 

 zling features. Near the line of Walcott's section the structure is plainly in- 

 dicated, but confusing unless very careful instrumental work be done. From 

 the summit of the Texan System at the north base of the mountain to the 

 top of the peaks and down upon the other side to the valley upon the south 

 there is no such simplicity as Mr. Walcott has described from a rapid recon- 

 noissance. That is not a good place to study the whole Cambrian System, 

 although it does afford some hints regarding the subsequent dynamic history 

 of our region. At the point of the westward extension of the Putnam 

 Mountains, southeast of House Mountain on the right bank of Hickory 

 Creek, there is the best possible exposure to explain what is not clearly ex- 

 hibited in many other contacts. Here the granite, of a type characteristic of 

 a Post-Cambrian (Silurian) uplift, has pushed up the conglomerate into a 

 southeastward dip rising towards House Mountain, and this is overlaid by the 

 set of conformable sandstones now recognized by myself as belonging to the 

 Lower Cambrian Hickory Series. These sediments have been mistaken 

 heretofore for the Potsdam sandstone, by the writer as well as others, but 

 in this section the Hickory Beds are unconformably overlain by a thick 

 series of strata which is itself unconformable below the Potsdam. From 

 their great development in the Riley Mountains the name Riley Series is pro- 

 posed for this set of beds, which presumably represents the Middle Cambrian 

 Epoch. It is not always easy to distinguish them from the overlying series, 

 although there is unconformity which can be made out in good exposures. 



The conditions prevalent during the Hickory Epoch were somewhat changed 

 in this Middle Cambrian Epoch, and the Riley deposits were laid down in 

 synclinal bays between the ridges whose elevation began in early Hickory 

 time. Probably that elevation continued during the Riley Epoch, for the 

 strata are chiefly such as are deposited in shallow water, their thickness im- 

 plying a gradual subsidence. The exposures indicate a different and more 

 irregular shore line than any previously existing in the region, but there 

 still was probably a considerable land area within our district. But an era 

 of subsidence had begun for the surrounding area, which did not cease until 

 a much later period than the Cambrian. 



Fine exposures of what I now believe to be the Riley Series occur in the 

 Sandy Watergap in the Riley Mountains; also over a wide area north of the 

 Llano and Brady road west of Valley Spring, as far as Fredonia and beyond, 

 spreading out through a large part of Mason County, where it is lost under 

 Cretaceous deposits. It reappears, as if by erosion of the Cretaceous, in a 



