136 .1. S. Mll.r.i: — SANDSTONE DIKES 



quake are well known. It is interesting to note thai the sand brought up 

 to the Burface at that time was, in Bome cases at 1 * -t i — t and perhaps in many, 

 decidedly micaceous, even more bo than thai in the sandstone dikes. 



Of the mineral particles usually found in Band the scales of mica are most 

 easily transported by water. This fad is sometimes made use of in petro- 

 graphic Laboratories to separate mica from other minerals in rock powders 

 by causing water to flow up through the rock powder, regulating the current 

 so that it will carry up the mica and allow it to escape above through an 

 outlet, while the other portion of the powder remains behind. The tendency 

 of this sort of action in Ailing earthquake fissures "would be to render the 

 Bands brought up to the surface more micaceous than those which remained 

 behind. 



The formation of a system of parallel fissures by earthquakes and filling 



them with Band forced up from below is a common phenomenon, and in all 



ntial features apparently identical with the formation of the Bandstone 



dikes described in this paper. It is reasonable, therefore, to regard these 



dike- as a record of ancient earthquake movement. 



Th> Region isfavorabli for the Production of sunk Phenomena. — The region 

 of the dikes is one of earthquakes, also ; and, when we consider its geologic 

 structure and compare it with that of countries where earthquakes have pro- 

 duced Buch phenomena, it is found to be well adapted to yield the same 

 results. Dolomieu's description of the country affected by the great earth- 

 quake of 1819 about the mouth of the Indus would in a general way answer 

 very well for the northwestern portion of the Sacramento valley. The 

 Cretaceous strata, as we have seen in the section figure 8, are bo situated as 

 to catch and hold great quantities of water flowing eastward from the Coast 



Range. Many of the streams sink in crossing the Creta< us belt, and the 



Bandstone beds, before they were indurated, must have been < pletely 



Bat u rated with water and ready to rush forth under the influence of an 

 earthquake t" till fissures in the sofl strata with -and. 



Source of the Sand in the Dikes. — It has been already remarked that cer- 

 tain sandstones of the Cretaceous bell are very Like those of the dikes. The 

 one to which there i- the greatest similarity is near the top of the cascade on 

 Byron creek, half a mile wesl of Ono. It is a stratum somewhat less than 

 LOO feet in thickness, with a strike N. LO E. and dip 20 toward the Sac- 

 ramento. Elsewhere its strike is more to the eastward, nearly parallel with 

 the western limit of the < Iretaceous terrane, and the average dip is about 15°. 

 The Bandstone bed outcrops about seven miles westward of the principal 



•up of dikes, and dips toward them at an angle ol 15 . [fits dips remain 

 constant as to direction and angle, as there i- reason to believe, and there is 

 no faulting, the bed must be in the neighbor! I of 10,000 feel below the 



