553 THE JOURNAL OF GEOLOGY. 



over ioo feet higher than the valley in which they stand, but 

 their serrated tops give them the appearance of a range of 

 eruptive hills. The beds here, unlike those at any other place 

 in the Texas Tertiaries, stand at high angles, and have a dip of 

 75 to 8o degrees to the southeast. 



The materials of which they are composed are claystones 

 interbedded with porcelaneous and siliceous rocks, partly flinty, 

 partly opaline, with bands and network of chalcedony and with 

 seams of ferruginous material. A few seams of calcite, in the 

 form of dog-tooth spar, and a bed of aragonite, 20 feet in thick- 

 ness, banded in brown and white and much knotted and twisted, 

 are found. The true opaline character of the rock was shown 

 by an analysis by Dr. Mellville, and the present condition may 

 be regarded as the result of infiltration of hydrous silica in hot 

 solution into the Tertiary marls, and their consequent alteration. 

 A number of specimens collected show that the marl was cracked 

 in every direction, and that these fissures are now filled with 

 chalcedony, while the marl is changed to a porcelaneous sub- 

 stance. 



The sands and clays of this division form the scarp known as 

 the Bordas, which forms the southern border of the Nueces 

 valley from Dinero to Los Angeles. It also caps many of the 

 outlying hills in the valley. 



Only a few fossils have been found, but such as are determin- 

 able — Protohippus medius Cope, P. perditus Leidy, .and P placidus 

 Leidy, Aphelops ineridianns Leidy, etc. — are sufficient to determine 

 its age as Loup Fork. 



The exact relation of these beds and those found in boring 

 the Galveston deep well has not been determined, since no 

 deposits containing similar marine Miocene fossils have been 

 found at the surface on the Coastal slope. The relationship of 

 the Deep Well Miocene and deposits of Florida and the West 

 Indies is shown by Harris 1 in his report on the organic remains 

 from that boring. 



On the Llano Estacado there is no great break between the 



1 Fourth Annual Report Geological Survey of Texas, p. 118. 



