Parr IL..§ vi] FRAGMENTAL ROCKS—PSAMMITIC. 158 
are products of the alteration of dolomite, the magnesia having: been 
taken up by silica, leaving the carbonate of lime behind as beds of 
limestone. It is conceivable, however, that in some cases at least, 
the serpentines were an original deposit from oceanic water, as 
has been suggested by Sterry Hunt in the case of those associated 
with the crystalline schists’ The beds of serpentine intercalated 
with limestone may have been due to the elimination of magnesian 
silicates from sea-water by organic agency, like the glauconite now 
found filling the chambers of foraminifera, the cavities of corals, the 
canals in shells and sea-urchin spines and other organisms on the 
floor of the present sea.2 Among the limestones and erystalline 
- schists of Banffshire serpentine occurs in thick Jenticular beds which 
-possess a schistose crumpled structure and agree in dip with the 
surrounding rocks. They may have been deposits of contempo- 
 raneous origin with the limestones and schists among which they 
occur, and in association with which they have undergone the 
characteristic schistose puckering and crumpling. 
B. FRAGMENTAL (CLASTIC). 
This great series embraces all rocks of a secondary or derivative 
origin; in other words, all formed of particles which have previously 
existed on or beneath the surface of the earth in another form, and 
the accumulation and consolidation of which gives rise to new 
compounds. Some of these materials have been produced by the 
mechanical action of wind, as in the sand-hills of sea-coasts and inland 
deserts (Holian rocks); others by the operation of moving water, 
as the gravel, sand and mud of shores and river beds (aqueous 
sedimentary rocks) ; others by the accumulation of the entire or frag- 
mentary remains of once living plants and animals (organic rocks) ; 
while yet another series has arisen from the gathering together of the 
loose débris thrown out by volcanoes (volcanic tuffs). It is evident 
that in dealing with these various detrital formations the degree of 
consolidation is of secondary importance. ‘The soft sand and mud 
of a modern lake-bottom differ in no essential respect from ancient 
lacustrine strata, and may tell their geological story equally well. 
No line is to be drawn between what is popularly termed rock and 
the loose as yet uncompacted débris out of which solid rocks may 
eventually be formed. Hence in the following arrangement the 
modern and the ancient, being one in structure and mode of 
formation, are elassed together. | 
It will be observed that in several directions we are led by the 
fragmental rocks back to those stratified deposits with which we 
began at p. 110. Both series of deposits are accumulated simul- 
taneously and are often interstratified; and, as we have seen, the 
1 Chemical Essays, p. 123. 
? According to Berthier, one of the glauconitic deposits in a tertiary limestone is a true 
serpentine. See Sterry Hunt, Chem. Essays, p. 308. 
