20 



nardino Mountains, as but the terminal portion of an auriferous belt, extending 

 nearly two hundred miles northward, and is again met with at the Armagosa 

 Mountains, a short distance from the immigrant road leading from Salt Lake to 

 San Bernardino. 



GEOLOGY OF TABLE MOUNTAIN, TUOLUMNE COUNTY. 



This celebrated locality, of which so much has been said and written of late, 

 is one of the most remarkable and interesting places to be found in this State. 



That portion of the mountain which has so recently attracted attention on 

 account of the placer deposits found beneath its surface, is situated near the 

 towns of Sonora and Columbia, being about equi-distant from either locality. A 

 portion of Shaw's Flat is situated immediately at its base. The superior portion 

 of the mountain is composed of basaltic rock having the prismatic form ; but in 

 a few instances, at different points, it assumes the pentagonal shape, so common 

 to the columnar form of this rock. 



I have traced this basaltic overflow in an east and west direction, for the 

 distance of nearly forty miles on its course, and it is probable that it will be 

 found to extend to a much greater distance eastward than at present known. 



As yet the origin of this volcanic outbreak is somewhat obscure, as no decided 

 volcanic vent is yet known to the east of this range, though such has been 

 reported ; nor is it necessary that a crater should exist to produce the features 

 that are throughout its extent observable. 



From all the evidence existing on the subject and in our possession at the 

 present time, it appears most probable that this immense mass of igneous mate 

 rial found its way to the surface through a large fissure produced from subterra 

 nean forces, and is in itself purely local. 



The vertical position of the columns prove, that for twenty miles of its course 

 at least, it must have flowed in a horizontal direction, and at the western end of 

 the bed, where it approached the plains and began to thin out, there we find it 

 following the accepted rule as regards the crystallographic forms assumed by this 

 rock. Although it is prismatic it loses to a certain extent its vertically, (as 

 may be seen at Peppermint Falls,) and the columns are inclined to various 

 degrees of the horizon. 



Its super-position is another proof of its horizontal movement, as well as its 

 comparatively modern age. The rocks over which it flowed on the south side of 

 the mountain are primitive in character, being composed of mica schist and 

 others of the granitic series, the former at the points of contacl having suffered 

 in texture and compactness from the heated mass. 



Its breadth is very variable throughout its course, in no instance, I believe, 

 less than four hundred feet and often to one-fourth of a mile. It seems to have 

 followed the course of a stream, filling its bed and banks, and to have flowed in 

 this course for the entire length that it is now observable upon the surface. 



To the east of Columbia it crosses the present bed of the Stanislaus at two 

 different places, but what its position beyond this may be is at present unknown 

 with any degree of certainty. From the relative position of the Stanislaus at 

 the present time the evidences are almost demonstrative, that at the period of its 

 occurrence it flowed into and down the former bed of this river, displacing the 

 latter and filling up the space between its banks. The depth of the banks to 

 the bed may be pretty accurately measured by the thickness of the basalt above 

 that bed, which is about one hundred feet on a general average. 



Subsequent to the deposition of these rocks the drift-banks of the stream have 

 been gradually removed, which has left the basalt in relief above them. A 



