326 



University of California Publications. 



[Geology 



hypothesis is borne out by the way in which the porphyry in 

 the vicinity of the Ruth Mine passes at comparatively low angles 

 beneath the Ruth limestone to the north, with considerable bodies 

 of blout along the contact. On the west of the same porphyry 

 mass the mapping of the blout is also very instructive, for there 

 it is revealed as a sheet of considerable thickness flanking the 

 porphyry, and extending from the summits of the hill down the 

 slope to the floor of Oeher Valley, where it passes beneath tbe 

 limestone. Mapping in other parts of the field shows the same 

 general relations. We may thus assume with considerable confi- 

 dence that the quartz blout is of the nature of an encasing shell 

 developed, so far as the field evidence goes, chiefly upon the 

 upper side of the porphyry laccoliths. It is, however, pretty 

 clear also from the failure of the blout to appear at certain local- 

 ities, where the contact of the porphyry and overlying limestone 

 may be mapped narrowly, that this shell was not perfectly con- 

 tinuous. It is also very probable from a study of various occur- 

 rences, that the shell of blout was not only a partial one, but 

 that it was of very variable thickness, and that the contact be- 

 tween the quartz and the porphyry was extremely irregular in 

 detail, the quartz often penetrating down into the porphyry 

 quite sharply. Some of this irregularity, however, may be ex- 

 plained by the excessive minor faulting, which as will be shown 

 later, affects the porphyry throughout. Faulting must also be 

 resorted to to explain certain subordinate occurrences of blout 

 in depressions in the surface of the porphyry below the general 

 level of the original shell, as for example, on the Josephine claim 

 to the northeast of the Ruth Mine. 



Besides these occurrences in immediate association with the 

 porphyry, in the relations above indicated, there are others of 

 minor importance as regards the scale on which they are devel- 

 oped, but which are nevertheless of much interest in throwing 

 light upon the question of the genesis of the quartz blout. These 

 occur not in the porphyry, nor on its contact with the limestone 

 but wholly in the limestone. Some of these, while they are now 

 quite disconnected from the porphyry, may have at one time 

 been connected with it. The quartz blout of the Twin Peaks, 

 for example, to the southeast of the Ruth Mine, may with much 



