500 
Proceedings of the Royal Soeiety 
materials, and the agents concerned in their deposition, distribution, 
and modification. 
Materials , — The materials which unite to form the deposits 
which we have to describe may be divided into two groups, viewed 
in relation to their origin, viz., mineral and organic. 
The mineral particles carried into the ocean have a different form 
and size, according to the agents which have been concerned in their 
transport. Generally speaking, their size diminishes with distance 
from the coast, hut here we limit our remarks to the mineralogical 
character of the particles. We find isolated fragments of rocks and 
minerals coming from the crystalline and schistomrystalline series, 
and from the clastic and sedimentary formations ; according to the 
nature of the nearest coasts they belong to granite, diorite, diabase, 
porphyry, S^o ,, ; crystalline schists, ancient limestones, and the sedi- 
mentary rocks of all geological ages, with the minerals which come 
from their disintegration, such as quartz, monoclinic and triclinic 
felspars, hornblende, augite, rhombic pyroxene, olivine, muscovite, 
biotite, titanic and magnetic iron, tourmaline, garnet, epidote, and 
other secondary minerals. The trituration and decomposition of 
these rocks and minerals give rise to materials more or less amorphous 
and without distinctive characters, but the origin of which is indi- 
cated by association with the rocks and minerals just mentioned. 
Although the debris of continental land to which we have just 
referred plays the most important role in the immediate vicinity 
of shores, yet our researches show beyond doubt that when we 
pass out towards the central parts of the great ocean basins, the 
debris of continental rocks gradually disappears from the deposits, 
and its place is taken by materials derived from modern volcanic rocks, 
such as basalts, trachytes, augitemndesites, and vitreous varieties of 
these lithological families, for instance, pumice and loose incoherent 
volcanic particles of recent eruptions, with their characteristic 
minerals. AU these mineral substances being usually extremely 
fine or areolar in structure, are easily attacked by the sea water at 
the place where they are deposited, This chemical action brings 
about an alteration of the minerals and vitreous fragments, which 
soon passes into complete decomposition, and in special circum- 
stances gives rise to the formation of secondary products. In some 
