530 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



Locally it contains red beds and lenses, but these are of the same 

 general character as the green clay rock in which they are inter- 

 bedded, except that they contain slightly more ir/m and alumina 

 and a little less silica. In certain beds there is a network of small 

 irregular joints, contiguous to which the greenish clay rock has 

 turned red; such a change seems to be due to the oxidation of the 

 greenish ferrous iron to the red ferric condition by surface waters. 

 In some of these red beds, however, there has been some local con- 

 centration of iron and alumina products. 



In addition to the red beds there are a few local beds and lenses 

 of gravel and of sandy, dark-gray, tufaceous material. This gravel,, 

 like the gravel at the base of the formation, is fairly fine, loosely 

 cemented, and consists of the rounded fragments of indurated shales, 

 cherts, and concretions from the lower part of this formation and 

 from some of the older rocks. 



There are also four distinct beds of lignitic shale, 1 to 5 feet thick. 

 They are the fossilized remains of former swamps. 



The formation is cut by some large and some small basaltic dikes, 

 but these have caused scarcely any metamorphism. Faulting has 

 considerably broken the beds and, owing to their soft and brittle 

 character, relatively small faults, where the movement seems to have 

 been less than 75 feet, have resulted in shear zones up to several feet 

 wide. These rocks weather readily, and are covered by 10 to 25 feet 

 of red soil. They are easily eroded, so that the outcrops of this 

 formation have mostly been worn into flats or valleys. 



Extending for more than a mile over what must have been an old 

 land surface, and now forming an interbedded unit of this forma- 

 tion, is a light to dark grayish, or, on fresh fracture, greenish, lava- 

 breccia flow of andesitic composition. Hand specimens of it show a 

 few little shiny faces of feldspar crystals up to 2 mm. in length, set 

 in a groundmass that resembles indurated clay. The brecciated frag- 

 ments are small, somewhat altered, and seem to have been picked 

 up from the formation over which the flow moved. Under the 

 microscope the rock is seen to consist of euhedral phenocrysts of 

 andesine ranging in size up to 1 by 2 mm. and some crystals of potash 

 feldspar set in a cloudy claylike groundmass, dark scaly areas result- 

 ing from the decomposition of some mineral, considerable chlorite, 

 some calcite, and a little secondary quartz. The outlines of the 

 brecciated fragments were recognized, but their original composition 

 was obscured by alteration. This altered andesite flow is somewhat 

 jointed and weak, so that it adds but little strength to the slopes 

 and is practically no protection against slides. 



The prevailing grayish-green color of the formation is due to the 

 fairly high percentage of very finely divided chloritic material that it 

 contains. These greasy mineral particles are a marked source of 



