14«2 
THE VOYAGE OF H.M.S. CHALLENGER. 
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CoNTI.VKSTAL AM> 
OcBANic Arras. 
tlie rocks of the contiuents. I'he bases arc being continually leached out of the rocks 
of the continents and carried awa)'’ in solution, the result being the deposition of the 
greater part of the heavier materials in the abysmal regions, and the accumulation of the 
greater part of the lighter insoluble and refractory quartz on or near the continents. 
Tlie average chemical composition of the pelagic or abysmal deposits shows only about 
36 j)er cent, of silica, while that of the terrigenous deposits and continental rocks shows 
about 68 per cent, of free and combined silica. Continental rocks have an average 
.specific gravity of about 2 ’5 ; the abysmal deposits would form rocks with a specific 
gravity of over 3d. In the original superficial crust all the silica was probably combined 
with bases ; the deeper layers would be more basic than the surface ones, but all the 
rocks of the crust were probably much less acid than granite. Gneisses and granites 
were probably formed subsequently to the commencement of subaerial denudation on 
the planet. ]\Iany considerations indicate that the continental masses are lighter por- 
tions of the Earth’s crust. Pendulum observations indicate a deficiency of mass beneath 
the continents. The plumb-line along continental coasts tend, it is said, towards the 
ocean basins. The grand result of all the denuding and reconstructing agencies in the 
past ajjpears to have been a great accumulation of quartz and highly siliceous materials 
on the present continental areas, that is, on the continents and the adjacent area.s 
covered by terrigenous deposits, and this is possibly the chief reason why the average 
level of the phine of the abysmal regions lies at about three miles beneath the plane 
representing the average height of the continents. 
If down to the close of Palmozoic times the ocean had throughout a nearly uniform 
high temperature, the dej)Osits then formed in deep water would certainly be different, 
for reasons indicated above, from what we now find in the abysmal regions. It is most 
probable that the. Ocean Basins were not so deep in these early ages, and numerous 
islands probably existed in them, with rocks similar to those that now make up the bulk 
of continental land. Possibly these former land-masses now form the submerged bases 
of the groups of oceanic islands wholly consisting, so far as we can see, of erupted 
rock.s. In the gradual evolution of the surface features of the planet, continental land 
ai^pears, on the whole, to have become more compact, more circumscribed and higher, 
while the ocean basins have become more shut oflf from each other and deeper. 
Continental land has been far from permanent, but there are many reasons for believing 
that the areas on the surface of the planet, within which the present continents are 
.situated, are areas within which continents have been torn down and built up again since 
the dawn of geological history, while similar revolutions have not taken place in 
abysmal or pelagic areas of the (Jeean Basins to anything like the same extent, and not at 
all during any of the later geological periods. In the evolution of the surface features of 
the globe and in the evolution of climate, as in the evolution of the solar system and in 
the evolution of organisms, there has been a progressive advance from sim[)le to complex 
«’omlition.s, from states of mf)re or less homogeneity to those of greater heterogeneity. 
