74 The American Geologist. Auguet, 1 05 
The sharp definition of this neck, with its continuous rim of 
granite, is, of course, chiefly due to the fact that erosion 
has removed the last vestige of the effusive felsite which 
we must assume to have once covered its site and a wide 
area of the enclosing granites. 
Not to classify this sharply defined body of acid lavas 
as a true neck or vent would seem to necessitate regarding 
it as a depressed fault block or graben, a remnant of a once 
widely extended volcanic sheet covering the granites which 
has escaped erosion through the accident of displacement. 
But the varied and prevailingly clastic character of the 
lavas and the general structure of the mass are, at least, 
highly suggestive of a vent, and force the conclusion that 
there must be a vent somewhere in this part of the com- 
plex. Although, omitting intersecting dikes of felsite, an- 
desite and diabase, the neck is almost wholly composed of 
effusive types of felsite ; it is yet, quite independently of 
the dikes, highly diversified in composition and structure — 
a constantly varying complex or chaos of fragmental, 
fluidal and compact or structureless lavas. The clastic 
phase, ranging from the finest tuff to the coarsest breccia 
or agglomerate, largely predominates and gives character 
to the whole, leaving little room to doubt that the later 
eruptions, at least, from this vent were in part explosive. 
Although varying greatly in azimuth and inclination, and 
usually much contorted, the normal attitude of the flow 
structure of the felsite is parallel with the axial plane of 
the neck, or, in the peripheral portions, with the proximate 
wall of granite. The interest of the peripheral phenomena 
culminates in the southeastern extremity of the neck, where 
it is continued in the shattered zone of granite in which it 
had its origin as a complex of granite and felsite, the 
granite being cut in all directions by irregular branching 
and coalescing dikes of the felsite, which is brecciated, 
banded, compact or porphyritic by turns and encloses many 
large and small angular fragments of the granite. 
While we need not dpubt that, the fragmental lavas 
are, in the main, true pyroclastics, the product of explosive 
eruptions, it is very probable that they are in part auto- 
clastics, or breccias resulting from the continued movement 
