310 Adventures in Scenery 



mountain systems. The rocks that we see today are the "roots," 

 so to speak, of the earlier mountain systems. Each of the an- 

 cestral mountain systems must have been in existence a very 

 long time for each was reduced to ridges and hills of only mod- 

 erate height. The time required for the wearing down of these 

 mountain systems is thought to have been between 50,000,000 

 and 100,000,000 years for each system. 



Roots of Two Mountain Systems and 

 Granitic Batholith Form Bed-Kock 



"The first of two ancestral mountain systems came into 

 being near the end of Palaeozoic time, more than 200,000,000 

 years ago. It was formed by the uplifting and folding of a 

 great series of layers of slate, shale, and sandstone, originally 

 mud, silt, and sand. Folded in with these sediments, thousands 

 of feet in thickness, were beds of lime, now metamorphosed to 

 marble. In the long streaches of time the wrinkles in the earth's 

 crust were in large part worn away, and finally the region sank 

 below sea level and sediments, together with beds of volcanic 

 material, were deposited upon the submerged remnants of the 

 first mountain system. Then, at the end of the Jurassic period, 

 about 130,000,000 years ago, there came another upheaval, and 

 the deposits which had been formed were folded and crumpled 

 and invaded by molten granitic magma from below. Thus a 

 second system of mountains arose. Throughout the Cretaceous 

 period, which followed the Jurassic, this second mountain 

 system was worn down to ridges and hills of moderate height. 

 The rocks which were deposited during the time of submer- 

 gence referred to, and were folded and crumpled during the later 

 mountain upheaval, are what make up the Mariposa formation. 



"It is in the folded and crumpled rocks of these two forma- 

 tions, the Calaveras and the Mariposa, that the gold-bearing 

 veins of the Mother Lode occur. The rocks of these two for- 

 mations, together with the lavas and injected granite of the 

 intruding batholith, make up what is called the Bed-rock series 



