60 
surface sheet of porphyrite and connected with the vents through 
which it was extruded. (2) That no dikes of this character 
have been discovered west of Green Hill or more than one mile 
ast of it. (3) That they all have, without exception, а 
northerly trend, č. e., they either run toward the Black Rock 
porphyrite or so as to intersect the eastward extension of its 
line of strike. And yet (4) none of them have been found 
breaking through the porphyrite itself, these dikes being thus 
comparable with the great melaphyr dike (No. 2), since in both 
cases the complete erosion of a portion of the effusive accumu- 
lations was essential to expose the intrusive masses — the roots 
of the eruptions — in the underlying granite. 
Green Hill Conglomerate.—The prominent Jedge of con- 
glomerate which has been laid bare by the wearing away of the 
northern slope of Green Hill, is probably, as the fault-lines of 
the map indicate, a part of the great bed forming Folsom's 
Island and Conglomerate Plateau; but it appears more con- 
venient to describe it in its geographic than in its geologic 
sequence. The ledge is about four hundred feet long from 
northwest to southeast, and shows throughout a southeasterly 
dip of 20° to 80°, giving an apparent thickness of perhaps 175 
feet. Beginning on the southeast, there are 10 or 12 feet of 
medium conglomerate with distinct layers of reddish sandstone, 
showing the bedding well. This is underlain by about 60 feet 
in thickness of very coarse conglomerate, many pebbles, espec- 
ially of granite, ranging from six to eighteen inches in diameter. 
The remainder of the section is conglomerate of a more normal 
character, varying from medium to rather coarse in texture. 
The conglomerate is composed throughout of well rounded 
masses of granite (coarse and pinkish), felsite of different vari- 
eties, and porphyrite; but, apparently, little true melaphyr. 
The porphyrite is mainly of compact, dark gray and brown va- 
rieties. 
