THE THINLY LAMINATED SLATKS. 431 



and slates of Grizzly ridge, along Spring Garden creek on both sides of it 

 above the American valley ( see sketch map, plate 13) : but farther north- 

 westward the serpentines are entirely absent and the older rocks brought 

 into direct contaet with tin,' slates by faulting* 



There is another narrow strip of serpentine on the souttnvestern side 

 of the slates at contact with the limestones and slates of the upper part 

 of the lower subgroup, on the left bank of the East branch of the North 

 fork of the feather. There are also small isolated patches of serpentine 

 on the faulted northern end of Claremont near limestone and slates of 

 the lower Mesozoic subgroup and on pre-Mesozoic rocks. As the Clare- 

 mont uplift dies out southeastward, hornblendic slates come in on the 

 northeastern side of the pre-Mesozoic exposure, which belong to the 

 serpentine series. 



Where the succession is uninterrupted .and where least interrupted by 

 faults the serpentine joins the slates and limestones at the head of the 

 lower subgroup. This is the ease for "JO miles along the line of ex- 

 posures of fossil iferous limestones before described. The slates at the 

 head of the Mesozoic series, for reasons to be hereafter given, may be 

 designated as the thinly laminated slates. Where the Mesozoic series is 

 complete or nearly complete the serpentines and slates which accom- 

 pany them lie between the thinly laminated slates and the rocks of the 

 lower subgroups. It is plain, therefore, that in the ascending series the 

 serpentines and the slates which accompany and replace them come 

 before the thinly laminated slates, and that the latter are at the head of 

 the whole series of metamorphic rocks of the Sierra. 



Serpentine. — Throughout the area between the North and Middle forks 

 of Feather river the lower part of the upper subgroup of Mesozoic 

 rocks is almost entirely of serpentine, although there are some schists 

 with it. and a part of these are glaucophanic. The schists may be 

 made up of lava transported and deposited by water wholly or in part. 

 South of the Middle fork the proportion of serpentine diminishes and 

 slates increase. These slates are much like those of the lower sub- 

 group and less thinly laminated than those at the head of the series. 



The serpentine is for the most part plainly (to the naked eye) a 

 product of alteration of a basic lava. The massiveness, cleavage and 

 absence of la in i i i;i t ;o; i or distinct plane-; of st ra t i licat ion all go 1" prove 

 this. M. E. Wadsworth describes, under the heading " peridotites," five 

 specimens of this rock from Sierra and Plumas counties within the dis- 

 trict next south of the one here more particularly treated of. and infers 



*Thi the western 1 gh is a line of recur- 



ring orographic n its, as shown by dislocations of Tertiary ■ I CJ ternary deposits and 



ury drain i 



