116 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
from attrition. The average attrition is generally small, far less than 
that of conglomerates in fossiliferous rocks (ibid., p. 75), and the rock 
grades into a breccia. The limited attrition may be explained by the 
scarcity of quartz sand in the detritus of the volcanic rocks. ‘The 
accumulation is not the product of volcanic explosions (ibid., p. 76) 
but is derived from the waste of a mass of volcanic rocks under the 
normal processes of degradation manifested. in mountain. regions. 
The material borne by the torrents through the ravines and gorges is 
spread out over the plains as depressed alluvial cones, so flat or so 
gently sloping that their conical form is not at first recognizable (ibid., 
p- 77-78). The rock is susceptible of a peculiar kind of metamor- 
phism. The matrix becomes similar to the included fragments, 
holds the same kind of crystals and under the microscope generally 
has the same texture and composition. Crystals are frequently seen 
lying partly in the original pebble and partly in the original matrix 
and the surface of fracture shows no inequality of hardness or cleay- 
age. The fragmental character of the material has disappeared (ibid., 
p. 79-80). 
In his discussion of the formation of conglomerates Dutton empha- 
sizes the relatively local nature of all conglomerates by the remark 
that “it would seem to require extraordinary circumstances to justify 
the belief that a conglomerate could be formed as far as fifty miles 
from the source of its fragments, and it is probable that most of the 
stratified beds are formed in the very neighborhood of those sources 
(ibid., p. 215). The proportion of coarse materials in one of the 
broad flat cones becomes greater toward the higher parts of the slope. 
The stream shifts its course on the cone and builds first one place, 
then another, but with approximately equal and uniform results. 
Sections along the radii of a cone give the best stratification. When 
transverse sections are cut the stratification, though still conspicuous, 
is much less uniform and harmonious. The cone appears built of 
long radial or sectoral slabs superposed like a series of shingles or 
thatches. Cones derived from the waste of sedimentary strata seldom 
contain coarse debris, while those of harder rocks are largely composed 
of it bid., p. 220-222). 
The “High Plains”, described by Johnson (p. 612) as uneroded 
remnants of an extensive debris apron that formerly extended 
along the eastern front of the Rocky Mountains, constitute another 
example of extensive fluviatile deposits in an arid region. ‘The apron 
has been formed by the coalescence of a series of alluvial fans along 
the mountain front (ibid., p. 615). The development of these fans 
