GEOLOGY OF THE SIEREA NEVADA. 



49 



t 



will be fully solved without much further research both in the field and in 

 the laboratory. The fact has already been stated that there is no assist- 

 ance to be obtained from fossils in deciphering the structure of the Sierra. 

 The belt of argillaceous slates described as occurring in Mariposa and Tuo- 

 lumne counties, in connection with the Great Quartz Vein, is undoubtedly of 

 Jurassic age ; and it may be assumed as highly probable that the finely 

 laminated slates farther north are the continuation of the southern belt of 



i 



similar lithological character. It may, perhaps, be considered as very prob- 

 able that a part or all of the limestone outcrops in the bed-rock series are of 

 Carboniferous age. But even then we are no farther advanced towards a 

 solution of the problem. The area of Triassic rocks in Plumas County covers 

 but a few acres, and is sharply bent at a right-angle. It is in close connec- 

 tion with the srranite, and is almost surrounded with volcanic. Not the 

 slightest clue can be obtained as to its relations with the associated members 



of the bed-rock series. 



It is evident from a number of facts developed during the Geological Survey, 

 that the rocks of the Sierra Nevada have undergone a large amount of longi- 

 tudinal compression, — that is to say, disturbance occasioned by forces acting 

 in the line of the strike of the formations. The manner in which the Triassic 

 outcrop in Plumas is bent sharply at right-angles, as already mentioned, is 

 a good illustration of this. A similar case is referred to, in Geology I., as 

 exhibited by the slates on the Mariposa Estate* The want of continuity in 

 the limestone masses, and the great and sudden changes of strike, as well 

 as in the form of the outcrops of this rock, point also to longitudinal com- 

 pression. Although the trend of the different members of the bed-rock series 

 is, on the whole, coincident with that of the axis of the Sierra, yet there are 

 many sudden and sharp dellections in their strike, which runs sometimes 

 north and south for a distance, and then again east and west, although the 

 average direction would be not far from northwest and southeast.! It is 

 true that a part of this apparent longitudinal compression may have been 

 caused by the unequally distributed force exercised by the irregular mass of 

 granite pressing in different directions according to its variations of form ; 

 but there are phenomena, in the structure of the rocks of the west slope of 

 the Sierra which seem only explicable on the theory of an actual thrusting 

 force exerted in the direction of their strike. 



* 



* See Geology of California, Vol. I. p. 225. 



f In one locality, at least, cast of Big Oak Flat, near Big Creek, the mica sktes have a strike of N. *70° - 

 "75° E., which direction is at an angle of 100° or more with the usual trend of the formation. 



