CHAP, in 
AGGLOMERATES, BRECCIAS AND TUFFS 
3 ' 
The chief lavas found in Britain. — Of the lavas which have been 
poured out at the surface within the region of the British Isles, the following 
varieties are of most frequent occurrence. In the acid series are llhyolites 
a-nd Felsites, but the vitreous forms are probably all intrusive. The inter- 
mediate series is represented by Trachytes and Andesites (Porphyrites), 
which range from a glassy to a holoerystalline structure. The basic series 
includes Dolerites, Diabases, Basalts, Limburgites (or Magma-basalts, con- 
faining little or no felspar), and Fieri tes or other varieties of Peridotites. 
The intrusive rocks display a greater variety of composition and structure. 
ii. volcanic agglomerates, breccias and tuffs 
The coarser fragmentary materials thrown from volcanic Agents are 
known as Agglomerates where they show no definite arrangement, and 
sspecially where they actually fill up the old funnels of discharge. MTien 
iiiey have accumulated in sheets or strata of angular detritus outside an 
f'Ctive vent they are termed Breccias, or if their component stones have 
keen water-worn. Conglomerates. The finer ejected materials may be com- 
prehended under the general name of Tuffs. 
Although these various forms of pyroclastic detritus consist as a rule 
of thoroughly volcanic material, they may include fragments of non-volcanic 
rocks. This is especially the case among those which represent the earliest 
explosions of a volcano. The first efforts to establish an eruptive vent 
lead to the shattering of the terrestrial crust, and the consequent discharge 
of abundant debris of that crust. The breccias or agglomerates thus pro- 
Tueed may contain, indeed, little or no tiuly volcanic material, but may be 
*iiade up of fragments of granite, gneiss, sandstone, limestone, shale, or 
'Whatever may happen to be the rocks through which the eruptive orifice 
has been drilled. If the first explosions exhausted the energy of the vent, 
h' may happen that the only discharges from it consisted merely of non- 
^oleanic debris. Examples of this kind have been cited from various old 
''oleauic districts. A striking case occurs at Sepulchre Mountain in the 
Yellowstone Park, where the lower breccias, the product of the earliest 
explosions of the Electric Peak volcano, and attaining a thickness of 500 
l®et, are full of pieces of the Arclnean rocks which underlie the younger 
formations of that district.^ These non-volcanic stones do not occur aimTiig 
fhe breccias higher up. Obviously, however, though most abundant at 
krst, pieces of the underlying rocks may reappear in subsequent discharges, 
''vherever by the energy of explosion, fragments are broken from the walls 
a volcanic chimney and hurled out of the crater. Illustrations of these 
features will be given in the account of the British Carboniferous, Permian 
Tertiary volcanic rocks. 
ft will be obvious that where the component materials of such frag- 
^‘lentary accumulations consist entirely of non-volcanic rocks, great caution 
be exercised in attributing them to volcanic agency. Two sources of 
’ Prof. J. P. Iddiiigs, XWiAnn . Rep . U . S . Geol . Survey (1890-91), p. 634. 
