134 CRETACEOUS ROCKS OF TEXAS. 



DISTURBANCES OP THE STRATA. 



It has been said in the previous pages that the rock sheets composing the 

 two Cretaceous series were comparatively uniform in their relative inclination 

 towards the sea, and that successively lower and lower (older) strata succeed 

 each other to the westward. In places the uniformity and continuity of 

 structure is somewhat disturbed by jointing, faulting, and denudation of the 

 strata. In the vicinity of Austin these joints and faults are especially con- 

 spicuous, no less than five of them being readily discerned between the east- 

 ern limit of the city and the top of Mount Bonnel. Their general direction 

 is north, 20 degrees east, but occasionally there are complemental directions to 

 these. The downthrow of these faults is usually to the east, but in two 

 exceptional cases it is reversed. The amount of this downthrow in 

 most of these faults is less than 100 feet, but that of the main one, which 

 runs from Mr. Huck's mansion parallel with the river, is over 500 feet, prob- 

 ably 750, so that the rocks underlying the region east of the Bonnell ridge 

 have fallen down from their former higher position. Accompanying the 

 downthrow of this fault are a large number of abnormal folds, usually dip- 

 ping in every direction, forming numerous low anticlines and synclines when 

 exposed. The rocks in the vicinity of these folds are usually metamorphosed 

 into marble, and their exact origin and relation to the proximate faults and 

 volcanic phenomena are seen seven miles southeast of the city, where the 

 uniformity of structure is again broken by an extrusion of basaltic rock at 

 Pilot Knob. 



The main, or Mount Bonnell, fault is one of the most conspicuous features 

 of the region, belonging as it does to a line of fracture extending from the 

 San Gabriel to the Bio Grande, and which in the southwest bears the appro- 

 priate name of the Balcones. This fracture in the earth's crust is marked by 

 an escarpment of from one hundred to two hundred and fifty feet in height, 

 the eastward face of which is usually composed of the basal, or alternating, 

 beds, while the summit, which is the plateau of the main Grand Prairie, is 

 composed of the hard Caprina limestone. The downthrow, or region to the 

 east, as seen in the country between Austin and Oatmanville, or between 

 Austin and Mount Bonnell, is more or less crumpled and broken by minor 

 accompanying fractures, accompanied by much metamorphism. 



