272 T. A. Bendrat — Rocks of Cei^ro de Santa Ana. 



lath-shaped, almost automorphic crystals of the feldspar are 

 frequently found projecting into the larger, formless crystals 

 of hornblende. Both the feldspars and the hornblendes have 

 apparently suffered much crushing and breaking, due to the 

 operation of dynamic forces, and like the types previously 

 described, the rock is penetrated by an intricate network of 

 delicate veinlets. The olivine however, which occurs in the 

 diabase, is wanting in this rock. 



Sandstone. — This occurs at the base of the mountain described 

 in the foregoing part of this paper. It is seen in thin beds, 

 slightly upturned toward the north and northeast and also as 

 debris in the arroyo issuing from the west slope, where it is 

 mingled with that of the diabase forming the base of the 

 mountain. It is a fine-grained, firm, reddish brown variety of 

 sandstone which is rich in muscovite. 



Studied in thin section, it is seen that the grains of quartz, 

 which is the chief component, are sharply angular and not 

 rounded like those met with in ordinary sandstones. Also, they 

 seldom appear to be in direct contact but are held together by 

 an interstitial cement in the form of nodular oolitic grains. 

 The quartzes have an undulatory extinction, indicating that 

 they were once components of some rock which suffered 

 dynamic processes. They are clouded with extremly fine 

 particles arranged in trains, which may be of magnetite, and 

 in some cases also needles of rutile. The muscovite occurs in 

 notched and jagged scales whose parallel arrangement is due 

 to sedimentation. It is more or less altered and associated with 

 some chlorite. Brown amphibole, plagioclase feldspar of 

 several varieties and microcline were also found in the rock. 

 The chemical test of the nodular oolitic cement, which was 

 kindly made for me by Dr. D. H. Neuland, proves it to consist 

 of siderite ; and it is considered by him to be a later infiltration 

 product. This makes the sandstone in question of interest, as 

 in most oolitic iron ores, according to Dr. Neuland, the oolites 

 enclose the quartz, as for instance in the Clinton iron ores. 



Spades, EipleyCo., Indiana, Dec. 1913. 



