
163 
varieties between rocks formed directly from the results of voicanic 
~ explosion and ordinary sedimentary deposits. 
Voleanic Conglomerate.—A rock composed mainly or wholly of 
rounded or sub-angular fragments of any volcanic rocks in a paste 
derived chiefly or wholly from the same materials, usually exhibiting 
a stratified arrangement, and often found intercalated between 
successive sheets of lava. Conglomerates of this kind may hays 
been formed by the accumulation of rounded materials ejected from 
volcanic vents; or as the result of the aqueous erosion of previously 
solidified lavas, or by a combination of both these processes. Well- 
rounded and smoothed stones almost certainly indicate long-continued 
water-action rather than trituration in a volcanic vent. In tke 
Western Territories of the United States vast tracts of country 
are covered with masses of such conglomerate, sometimes 2000 feet 
thick. Captain Dutton has recently shown that similar deposits are 
in course of formation there now, merely by the influence of dis- 
integration upon exposed lavas." 
Volcanic conglomerates receive different names according to the 
nature of the component fragments ; thus we have basalt-conglomerates, 
where these fragments are wholly or mainly of basalt, trachyte-con- 
glomerates, porphyrite-conglomerates, phonolite-conglomerates, We. 
Voleanic Breccia resembles volcanic conglomerate, except that 
the stones are angular. This angularity indicates an absence of 
- aqueous erosion, and, under the circumstances in which it is found, 
usually points to immediately adjacent volcanic explosions. There 
- isa great variety of breccias, as basalt-breecia, diabase-breccia, &e. 
Voleanie Agglomerate.—A tumultuous assemblage of blocks of 
all sizes up to masses several yards in diameter, met with in the 
- “necks” or pipes of old voleanic orifices. The stones and paste are 
commonly of one or more voleanie rocks, such as basalt or porphyrite, 
but they include also fragments of the surrounding rocks, whatever 
these may be, through which the volcanic orifice has been drilled. 
As a rule, agglomerate is deyoid of stratification; but sometimes it 
includes portions which have a more or less distinct arrangement into 
beds of coarser and finer detritus, often placed on end, or inclined in 
different directions at high angles, as described in Book LY. Part VII. 
Voleanic Tuff.—This general term may be made to include all 
the finer kinds of volcanic detritus, ranging on the one hand through 
coarse gravelly deposits into conglomerates, and on the other into 
exceedingly compact fine-grained rocks formed of the finest and 
most impalpable kind of volcanic dust. Some modern tuffs are full 
of microliths derived from the lava which was blown into dust. 
Others are formed of small rounded or angular grains of different 
lavas, with fragments of various rocks through which the volcanic 
funnels have been drilled. The tuffs of earlier geological periods 
have often been so much altered, that it is difficult to state what may 
have been their original condition. The absence of microliths and 
1 High Plateaux of Utah, p. 77. 
M 2 
