86 
of about 25°; and this is still farther augmented by a fault with 
a downthrow to the south of perhaps ten feet, which skirts the 
northern edge of the porphyrite and crosses the level platform 
of conglomerate to the western base of the hill, as shown on the 
map, thus explaining the abrupt termination of the melaphyr 
midway of the breadth of the hill. Along the contact with the 
conglomerate the porphyrite, as on the eastern hill, is some- 
what brecciated, and shows a distinct flow-structure parallel with 
the contact, as well as a coarse sort of spherulitic or concretionary 
structure. On the southwest corner of the hill the conglomer- 
ate is seen to pass beneath the porphyrite, but the contact is 
not clearly exposed. The southern face of the porphyrite is 
precipitous, matching the opposing face of Cliff Plateau ; and 
there can be no reasonable doubt that this narrow pass marks 
an important fault with the downthrow to the north ; for the 
porphyrite is vis à vis with the basal conglomerate, and the 
melaphyr exposed at the northwestern base of West Porphyrite 
Hill is the equivalent of that forming the southeastern summit 
of Cliff Plateau. The displacement must, be equal to the com- 
bined thickness of the porphyrite and the underlying conglom- 
erate and melaphyr, 2.e., all the beds in West Porphyrite Hill, or 
75 feet as а minimum. This fault probably passes southwest 
between the outlying ledges of conglomerate on the north and 
granite on the south; and the boundary fault between Granite 
Plateau and Cliff Plateau, which certainly diminishes rapidly 
in this direction, appears to terminate on this line, as repre- 
sented on the map. 
We now have above the granite, in regular sequence, the 
following beds: (1) the first or basal conglomerate, on Granite 
Plateau, Granite Point, and Cliff Plateau; (2) the first 
melaphyr, on Cliff Plateau and East and West Porphyrite 
Hills; (3) the second conglomerate, on Cliff Plateau and 
East and West Porphyrite Hills; (4) the porphyrite, on the ' 
Porphyrite Hills ; and (5) the third conglomerate, on the south- 
east corner of East Porphyrite Hill. From Granite Point to 
