Parr Il. § vi] FRAGMENTAL ROCKS—PSAMMITIC, 157 
granite-conglomerate, &c. or according to that of the paste or 
cementing matrix which may consist of a hardened sand or clay, 
and may be siliceous, calcareous, argillaceous, or ferruginous. In the 
coarser conglomerates, where the blocks may exceed six feet in 
length, there is often very little indication of stratification. Except 
where the flatter stones show by their general parallelism the rude 
lines of deposit, it may be only when the mass of conglomerate is 
taken as a whole, in its relation to the roeks below and above it, 
that its claim to be considered a bedded rock will be conceded. 
The occurrence of occasional bands of conglomerate in a series of 
arenaceous strata is analogous probably to that of a shingle bank 
or gravel beach on a modern coast-line. But it is not easy to under- 
stand the circumstances under which some ancient eonglomerates 
accumulated, such as that of the Old Red Sandstone of eentral 
Scotland, which attains a thickness of many thousand feet, and 
-eonsists of well rounded and smoothed blocks often -several feet in 
diameter. 
In many old conglomerates (and even in those of Miocene age 
in Switzerland) the component pebbles may be observed to have 
indented eachother. In-such cases also they may be found split and 
recemented; sometimes the same pebble has been crushed into a 
number of pieces, which are held together by a retaining cement. 
These phenomena point to great pressure, and some internal relative 
movement in the rocks. (Book III. Part I. Section iv. § 3.) 
Breccia.—A rock composed of angular instead of rounded 
fraements. It commonly presents less trace of stratification than 
conglomerate. Intermediate stages between these two rocks, where 
the stones are partly angular and partly subangular and rounded, 
are known as breeciated conglomerate. Considered asa detrital deposit 
formed by superficial waste, breccia points to the disintegration of 
rocks by the atmosphere, and the accumulation of their fragments 
with little or no intervention of running water. Thus it may be 
formed at the base of a cliff either subaerially, or where the débris of 
the cliff falls at once into a lake or into deep sea-water. 
The term Breccia has, however, been applied to rocks formed in 
a totally different manner. Intrusive igneous masses have sometimes 
torn off fragments of the rocks through which they have ascended, 
and these angular fragments have been enclosed in the liquid or 
pasty mass. Or the intrusive rock has cooled and solidified externally 
while still mobile within, and in its ascent has eaught up and in- 
volved some of these consolidated parts of its own substance. Again, 
where solid masses of rock within the crust of the earth have ground 
against each other, as in dislocations, angular fragmentary rubbish 
has been produced, which has subsequently been consolidated by 
some infiltrating cement (fault-rock). It is evident, however, that 
breccia formed in one or other of these hypogene ways will not as a 
rule be apt to be mistaken for the true breccias, arising from super- 
ficial disintegration. 
