Relations of the Igneous Rocks — Crosby. 75 
of the magma after it began to stiffen and solidify. The 
distinction of these two types in the field must usually be 
a puzzling matter, so much depending on the nature of the 
matrix or ground mass ; but it is assumed here that local 
homogeneity of composition (not of texture) and absence 
of granitic debris are characters belonging more normally 
to the autoclastics. In considering the distinction and 
probable relative abundance of the pyroclastic and auto- 
clastic lavas, both of which are undoubtedly prominent 
features of this neck, and may also be designated, respec- 
tively, agglomerate and breccia, we may properly take ac- 
count of the possibility that the eruptions were subaqueous, 
at least in part, and note the cracking and shattering of 
the lava that would follow its sudden quenching. It is 
doubtful, however, if much of the breccia has the crackled 
character, with accurately fitting fragments, which this 
explanation would require. Nevertheless, the presence of 
water is plainly suggested by the rather distinct stratifica- 
tion of some of the finer tuff's, such as may be observed in 
the central part of the neck especially. 
Attention is thus directed to one of the most interest- 
ing and puzzling features of the neck. This is a very com- 
pact gray, slaty-looking rock, irregular masses of which are 
enclosed in the more normal or unquestionable agglomer- 
ate and also in the fluidal felsites and occur only in the 
western central portion of the neck, all the outcrops being 
included within an area about one thousand feet long in 
a north-south direction and perhaps half as wide. This 
material, which may be as distinctly, evenly and finely, or 
as obscurely, stratified as any slate, was at first mistaken 
for an older slate enclosed in the volcanics. But further 
study of the field relations showed that it must be contem- 
poraneous and essentially a tuff or consolidated ash. As 
noted by Dr. Bascom, this conclusion is confirmed by both 
the microscopic and the chemical analyses. The combined 
alkalies are far higher than for a normal slate and agree 
closely with those of the* felsites. These masses range in 
extent from a few inches to many feet. They are some- 
times ill-defined, cloud-like patches in the lavas; but more 
commonly they are sharply outlined and the contacts with 
