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are laid down upon the bottom of the sea. This assemblage of pyro- 
genic rocks, rich in aluminous silicates, decomposes under the chemical 
action of the water, and gives rise, in the same way as do terrestrial 
volcanic rocks, to argillaceous matters, according to reactions, which 
we can always observe on the surface of the globe, and which are 
too well known to need special mention here. 
The detailed microscopic examination of hundreds of soundings 
has shown that we can always demonstrate in the argillaceous matter 
the presence of pumice, of lapilli, of silicates, and other volcanic 
minerals in various stages of decomposition. 
As we have shown in another paper,* the deposit most widely 
distributed over the bed of modern seas is due to the decomposition 
of the products of the internal activity of the globe, and the final 
result of the chemical action of sea water is seen in the formation of 
this argillaceous matter, which is found everywhere in deep-sea 
deposits, sometimes concealed by the abundance of siliceous or cal- 
careous organisms, sometimes appearing with its own proper charac- 
teristics associated with mineral substances, some of which allow us to 
appreciate the extreme slowness of its formation, or whose presence 
corroborates the theory advanced to explain its origin. 
In the places where this red clay attains its most typical develop- 
ment, we may follow, step by step, the transformation of the volcanic 
fragments into argillaceous matter. It may he said to he the direct 
product of the decomposition of the basic rocks, represented by vol- 
canic glasses, such as hyalomelan and tachylite. This decomposi- 
tion, in spite of the temperature approximating to zero (32° P.), gives 
rise, as an ultimate product, to clearly crystallised minerals, which 
may he considered the most remarkable products of the chemical 
action of the sea upon the volcanic matters undergoing decom- 
position. These microscopic crystals are zeolites lying free in the 
deposit, and are met with in greatest abundance in the typical red 
clay areas of the central Pacific. They are simple, twinned, or 
spheroidal groups which scarcely exceed half a millimetre in 
diameter. The crystallographic and chemical study of them shows 
that they must be referred to Christianite. It is known how easily 
the zeolites crystallise in the pores of eruptive rocks in process 
of decomposition ] and the crystals of Christianite, which we 
* “On Cosmic and Volcanic Dust,” Proc. Roy. Soc. Edin., 1883-84. 
