ILLUSTRATIONS OF GEOLOGICAL STEUCTUEE. 163 



traced through the entire series of lacustrine beds. The occurrence of 

 two distinct and well-marked systems of joints in a bed of marly clay 6 

 feet thick, lying between unconsolidated gravels, has been noticed on 

 page 132. This may be taken for an example of what may be seen at 

 numerous localities. The most striking exhibition of jointing that we 

 have observed in the Lahontan strata occurs in the upper clays on the 

 west side of the Humboldt River, near Saint Mary's. An arroyo has there 

 exposed a vertical cliff 25 feet high, of homogeneous, marly clay that is cut 

 from top to bottom by joints which divide the material into small pentag- 

 onal prisms that bear a superficial resemblance to basaltic columns. Speci- 

 mens of these prisms may be collected that are 2 or 3 feet in length and 

 not over 3 or 4 inches in diameter, with sharply-defined edges ; in some in- 

 stances the diameter of the columns is much less than here indicated, the 

 prismatic form being still well preserved. The jointing of the Lahontan 

 sediments is of the same nature as the similar phenomena observed in the 

 Bonneville basin, the origin of which has been the subject of some dis- 



cussion.^^ 



FAULTS. 



Two systems of faults have affected the sediments of Lake Lahontan ; 

 the first is of wide extent and due to a recent movement along the ancient 

 lines of displacement which gave origin to the structural features of the 

 region ; the second is of local origin, and seems to be entirely independent 

 of orographic disturbances. Displacements of the first class will be de- 

 scribed in a future chapter devoted to the description of post-Lahontan oro- 

 graphic movements. The local faults in which we are interested at present 

 are common in the soft, unconsolidated sediments of the ancient lake, but 

 even in the most typical instances their displacement does not exceed a few 

 feet, and, as indicated by several observations, they appear to have small 

 vertical range, i. e., their throw diminishes and finally disappears when 



5' G. K. Gilbert, " Post-Glacial Joints," American Journal of Science, Vol. XXIII, 1882, pp. 25-27. G. 

 K. Gilbert, " On the Origin of Jointed Structures," American Journal of Science, Vol. XXIV, 1882, p. 50. 

 John Le Conte, "Origin of Jointed Structure in Uudisturbcd Clay and Marl Deposits," American Jour- 

 nal of Science, Vol. XXIII, 1882, p. 233. W. 0. Crosby, " On the Classification and Origin of Joiut-Struct- 

 ure," Proceedings Boston Soc. Nat. Hist., Vol. XXII, 1882, pp. 72-85. H. F. Walling, " On the Origin of 

 Joint Cracks," Proceedings American Association for the Advancement of Science, Vol. XXXI, 1882, p. 

 417. 



