1914] 



Dickerson: Eocene of the Santa Ana Mountains 



261 



BASEMENT COMPLEX 



The basement complex is composed of a series of slates, quartzites, 

 and limestones of probable Triassic age, which have been extensively 

 intruded by granites, by dikes of rhyolite and other acidic intrusives 

 and mantled over by lava flows. The quartzites and schists have not 

 been sufficiently metamorphosed to obscure their bedding planes, and 

 on the western flanks of the range they dip, in general, to the northeast 

 at angles of 45 to 80 degrees. This basement complex is unconform- 

 able' overlain by the Chico. These rocks are described by Mendenhall 7 

 as follows : 



The Santa Ana Mountains, usually regarded as a southern extension of the 

 Coast Ranges, form a portion of the boundary between Riverside and Orange 

 counties in southern California. The group, which lies for the most part south 

 of the lower course of Santa Ana River and west of the Temescal Wash, 

 culminates in Santiago Peak, 5680 feet high. Its axis is made up of a series 

 of dark-gray or black slates with minor amounts of interbedded brown sand- 

 stones, the whole sparingly intruded by a series of medium acidic dykes and 

 overlain unconformably by remnants of the associated effusives whose aspect 

 is generally that of andesites or slightly more acidic rocks. 



The slates exhibit varying degrees of metamorphism. They usually have a 

 well-developed cleavage, which, however, is generally not sufficiently perfect 

 to obscure the original bedding planes. In general appearance they resemble 

 the Mariposa slate of central California, although as a rule they are less 

 extensively altered. These sediments are the oldest rocks of the mountain 

 range in which they occur. The effusives already mentioned overlie the slates, 

 but have been affected by a part of the same metamorphism. 



Both the sediments and the associated effusives have been intruded and 

 slightly altered by great masses of granitic rocks, and this threefold series 

 after a long time-interval, represented by an extensive physical unconformity, 

 has been at least partly buried under Cretaceous conglomerates and shales 

 of Chico aspect that are now entirely unaltered though extensively deformed. 

 These upper Cretaceous rocks form an encircling outcrop that flanks the dome 

 of older rocks. 



The determination of the age of the slates is based on small collections 

 made in Ladd Canon, on the south slope of the range, and near the mouth of 

 Bedford Canon, on its north slope. These collections were examined by Dr. 

 Stanton, who reports as follows on the Bedford Canon collection: 



' ' The two Triassic lots, both from the neighborhood of Bedford Canon, evi- 

 dently came from essentially the same horizon. No. 230 contains fine specimens 

 of a large species of Bhynconella of a Mesozoie type and a single specimen of 

 Spiriferhia. No. 321 contains the same species as 230 and in addition a plicate 

 form of Terebratula and fragments of erinoid stems. These fossils taken to- 

 gether clearly indicate the Triassic age of the fauna, but in the absence of 

 ammonites and other diagnostic forms it is not possible to determine the exact 

 horizon, although it is probably upper Triassic rather than older." 



7 Mendenhall, W. C, in Index to the stratigraphy of North America, U. S. 

 Geological Survey, Professional Paper, no. 71, p. 555, 1912. 



