Vol. 4] Lawson. — The Robinson Mining District. 



313 



is of two kinds: (1) the structural relations of the porphyry 

 mass to the surrounding' strata; and (2) the contact metamor- 

 phistn displayed at several points on the margin of the porphyry. 



Structure. — The structural evidence of the intrusion of the 

 porphyry is perhaps most apparent in the eastern area and 

 particularly in the vicinity of the Ruth Mine. Here the por- 

 phyry belt lies transverse to a well marked northerly pitching 

 syneline of the Ruth limestone. On the south side of the por- 

 phyry belt different stratigraphic horizons of the Ruth limestone 

 abut upon it and the plane of contact does not conform to the 

 planes of stratification, but in general appears to dip under the 

 porphyry. On the north side of the belt, the Ruth limestones 

 overlie the porphyry, dipping away from it but again meeting 

 it at different stratigraphic horizons. It is evident from this 

 relationship not only that the porphyry is intrusive in the Ruth 

 limestone, but, also, that the intrusion occurred subsequent to the 

 establishment of the synclinal structure. 



The way in which the upper surface of the porphyry passes 

 beneath the limestone at low angles on the north side of the 

 belt, the strata dipping away from the intrusive mass, suggests 

 strongly that the structure is, in general, for this part of the belt, 

 laccolithic. On both flanks of the Ruth syneline the porphyry 

 comes in contact with the underlying Arcturus shaly limestone. 

 On the west flank it appears to pass beneath the Arcturus for- 

 mation although cutting across various horizons. On the east 

 it passes over upon the Arcturus formation. 



On Jupiter Ridge and from thence eastward to the Chain- 

 man Mine, the mapping of the porphyry indicates that it lies 

 upon the White Pine shale as a warped sheet, partly removed 

 by erosion, but cutting across the contact of the shales with the 

 overlying Ely limestone. On the lower flanks of this slope at 

 the upper end of Lane Valley, it is mantled by the remnants of 

 a roof of limestone and it probably sustains a similar relation 

 to two other areas of limestone between this and the Chainman 

 Mine although this relationship was not established in the field. 



In the western area of porphyry the laccolithic character of 

 the intrusion is even more apparent. Along the eastern edge 

 of this area, in the vicinity of Copper Plat, the upper surface 



