106 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
wacke. The relative proportions of pebbles and matrix may also vary 
from a rock consisting of practically nothing but boulders to a pebbly 
sandstone. Variations in bedding may also occur so that strata may 
suddenly sink from a thickness of several hundred feet to a few yards, 
or may die out altogether, perhaps to reappear in the same wedge-like 
fashion. Conglomerates are thus, as Geikie well says (ibid., p. 651), 
“the most variable and inconstant of all sedimentary formations,” 
but whatever irregularities may occur the pebbles maintain some 
degree of rotundity. 
Breccias. Allied to conglomerates and sometimes shading into 
them are rocks composed of angular instead of rounded fragments 
breccias. They commonly present less marked stratification than 
conglomerates and have been only slightly, if at all, affected by run- 
ning water. Various agencies may produce breccias, which are named 
according to their mode of origin. The normal breccia is formed by 
the accumulation of the products of the superficial decay of rocks, 
without the intervention of water action, save that involved in the 
rapid deposition of material launched from steep slopes or cliffs into a 
lake or the deep sea. Angular blocks and fragments accumulated 
around vents may become consolidated into volcanic breccias. Igneous 
eruptions may tear off fragments of the rocks through which they pass 
or may break through previously consolidated portions of the igneous 
rock, thus forming, when the mass solidifies, igneous breccias. Move- 
ments of rock masses along fracture planes sometimes produce a zone 
of angular fragments which, by subsequent infiltration and deposition 
of mineral matter, becomes a fault breccia (ibid., p. 164). 
Agglomerates. Tumultuous assemblages of blocks, of all sizes 
up to masses several yards in diameter, occur in the necks or pipes of 
old volcanic openings. The stones and paste usually consist of one or 
more volcanic rocks; but they may sometimes include fragments of 
the rock through which the orifice was drilled. Such accumulations 
are called volcanic agglomerates. As a rule they are devoid of strati- 
fication but they may sometimes include coarser and finer materials 
arranged in more or less distinct beds, which are often placed on end 
or inclined at a high angle. The last feature is probably due to the 
subsidence of beds of tuff within the cr: rater and upper part of the 
funnel (ibid., p. 174, 751). 
Volcanic Conglomerates. When the coarser volcanic fragments 
become rounded by attrition within the crater or funnel, and especially 
when they are resorted and deposited by moving water, the consoli- 
dated accumulations formed are called volcanic conglomerates (ibid., 
p- 276). 
