408 
THE VOYAGE OF H.M.S. CHALLENGER. 
the submarine zeolites. Keeping in mind the whole range of facts furnished by the 
study of zeolitic rocks, and of the formation of contemporary zeolites, we may proceed to 
consider the origin of the little crystals of phillipsite from the clays of the Pacific. The 
dredgings and soundings in these zeolitic regions show an exceptional abundance of frag- 
ments or of lapilli of vesicular basalt, often with a highly-developed vitreous base. 
Almost all these rocks belong to the basic series, and among them are found types of the 
eruptive series poorest in silica. With these lapilli, which are always observed in a state 
of hydration and more or less advanced disaggregation, are associated, with remarkable 
constancy, fragments of palagonite representing one of the last phases in the hydration of 
basaltic volcanic glasses. Microscopic particles also observed in these clays must have 
been projected as volcanic ashes from submarine or subaerial eruptions, and have appa- 
rently come from eruptions that have covered the bottom of the sea with eminently 
alterable lapilli, similar to those just referred to. These particles are also generally of a 
basaltic nature, and their state of extreme division must render them in an exceptional 
degree favourable for attack by sea-water. 
It is seen, then, not only that the rocks just enumerated are those in which, in 
geological formations, zeolites have been developed in a marked degree, but that they 
are especially represented by the vitreous varieties. Moreover, it is known, from 
observations of geological formations, and from experiments in the laboratory, that these 
vitreous varieties are precisely those which, as might be expected, ofier the least 
resistance to the action of water, and that water transforms them, in part at least, with 
great facility into matters of a zeolitic nature. What may be expected to be the result 
of the action of water upon the rocks and minerals found on the bed of the Pacific ? 
Evidently the same at the bottom of the sea as that observed in analogous rocks on the 
dry land, where we are able to follow the modifications there taking place. As we have 
already remarked, the minerals constituting the basalts and basaltic rocks in general 
undergo, under the influence of waters that attack them, a series of transformations pro- 
duced with constancy in nature, which may be thus summarised. During the decom- 
position of these rocks the waters take away from the alkaline silicates almost all they 
contain of potash and soda, silica being at the same time liberated ; in silicates with a 
base of lime, magnesia, iron, and manganese, almost the whole of the lime and of the 
iron is separated with a notable quantity of silica. These various elements tend to 
disappear from the primitive mass, being taken away by the waters, but sometimes the 
iron and the manganese remain in the residue of the decomposition in a high state of 
oxidation. As to the alumina entering into the composition of these silicates, a 
frarttion of it is eliminated, but the greater part is concentrated in the residue, in 
retaining a certain portion of the silica, and in fixing a certain quantity of the 
water. The final product of this decomposition approaches more and more to a hydrated 
silicate of alumina, which constitutes an argillaceous mass containing always traces of 
