262 University of California Publications in Geology [Vol. 8 



Thus the collections, although meager, seem abundantly sufficient to estab- 

 lish the Triassic age of the slates. Accepting this age, then, as determined, 

 we must assign the later granitic intrusions to the Jurassic or the ear]}' 

 Cretaceous. 



CHICO GEOUP 



Mr. Earl L. Packard contributes the following description of the 

 Chico group : 



The Chico outcrops on the western flank of the range in a band about three 

 and one-half miles wide at its maximum width, tapering to a point at a distance 

 of about thirteen miles to the southeast. On the eastern side of the range the 

 strip is narrower, being about a mile in width and three in length. The com- 

 bined areas of Cretaceous outcrops would probably amount to thirty-five square 

 miles. 



The contact with the "complex'' may be seen best in Harding Canon, where 

 the metamorphic series of sandstones has a strike of about N 15° W and a dip 

 of 45° N. Contrasted with this is the strike of about N 20° W and a dip of 

 25° S, taken upon the Chico sandstones just above the basal conglomerates. 

 In places tne low dip of the Chico carries the red basal conglomerates some 

 distance up the slopes of the ridges into the area of the "complex", in such 

 a way that there are isolated patches of small extent, separated by erosion 

 from the main mass, or connected with it by a thin veneer of residual gravels. 



The beds exposed on the western flank are possibly the remnants of one 

 limb of an asymmetric anticline, the eastern limb of which has been broken 

 by a fault extending along the base of the range. The more gently westward- 

 dipping beds have a strike of about N 45° W and a dip of 25° to 30°. This 

 holds true in general from the southern extent of the outcrops northward to 

 Black Star Canon. On the west side of this canon the beds are gently in- 

 clined, the dip in places not exceeding ten or fifteen degrees. Still farther 

 north the structure becomes complicated by minor gentle folds. Northeast of 

 Coal Mine Hill a carbonaceous stratum marks one limb of a small anticline. 

 Coal Mine Canon, north of Sierra Canon, is nearly parallel to the axis of a 

 pre-Tejon fold which involved the Martinez and Chico strata. On the east 

 side of the range Cretaceous fossils were obtained north of Tin Mine Canon 

 in strata the dip and strike of which varied greatly within short distances. 

 This and certain physiographic forms demonstrate a fault extending along the 

 eastern base of the range. 



The rocks of the series range from a heavy conglomerate through several 

 grades of sandstone, to fine shale often bearing nodular limestones. An almost 

 ideal section of the entire series is exposed in Silverado, Williams, and Harding 

 canons. 



The base of the Chico group consists of a conglomerate of characteristic 

 dark-red color, composed of boulders formed chiefly from basic, igneous rocks 

 ranging up to a diameter of two feet, all rather loosely cemented by a red 

 sandy matrix. In places cross-bedding occurs. The rock weathers easily, form- 

 ing gentle slopes in marked contrast to the perpendicular cliff's characteristic 

 of the overlying gray conglomerates. These conglomerates probably represent 

 a stream-laid deposit. [See plate 27, figure 1.] 



A gray conglomerate lies upon the lower red conglomerate. At some localities 

 red shaly sandstones of the basal member are intercalated as distinct bands 

 with the gray conglomerate above. The latter differs from the lower member 



