406 
THE VOYAGE OF H.M.S. CHALLENGER. 
developed as secondary minerals in the hollow cavities, the vesicles, the fissures, of some 
older or recent eruptive masses and in their tufas ; they are sometimes also seen 
pseudomorjdiosed on anhydrous silicates. An intimate bond unites the zeolites with 
the matters upon which they are implanted or with which they are associated. It might 
be said that these hydrated silicates are nothing else than the volcanic minerals trans- 
formed under the action of water and in a manner regenerated ; as soon as these crystal- 
line rocks or their tufas are exposed to the action of water that penetrates them their 
j)orcs are seen to be lined with zeolitic minerals. This filling up is in direct relation 
with the degree of alteration of the rocks ; in short, these veins and geodes have 
been lined by zeolitic minerals by an exudation, so to speak, of the rock containing 
them. It is especially in the geodes of basalts, of phonoliths, of diabases, or in the 
respective tufas, that they are met with. The submarine volcanic matters of the 
regions already indicated are precisely those that might be considered as the tufas of 
basaltic rocks. 
The study of the crystals and zeolitic coatings lining the cavities of products of 
subaerial eruption indicates clearly that these secondary minerals have been formed by 
waters, which have taken from the very rocks through which they have passed the 
constituent elements of the zeolites. We may even follow in the various zones of the 
geodes the gradual series of alterations that the rocks have undergone under the influence 
of the infiltrating water ; it has deposited in the hollow cavities matters with which it 
has been gradually charged during its passage through the capillary canals traversing 
these eruptive masses. Amygdaloid rocks of the basic series of all geological formations 
exhibit the conditions here recalled ; it has even been shown that, in lavas so recent 
as those of the Puy-de-D6me and of Gravenoire, these zeolites are present. In a word, 
wherever basic volcanic rocks are exposed we are sure to observe minerals belonging to 
the group of zeolites, always formed by the solvent action of waters upon the volcanic 
masses containing them. This is the case in Auvergne, in Bohemia, in the Siebengebirge, 
in the rocks in the neighbourhood of Idar, in Iceland, in the Deccan, in the eruptive 
masses in the Trias of Scotland, &c. 
It is only in exceptional circumstances that the zeolites are observed in sedimentary 
layers. The solutions depositing them may then have taken the elements from the 
neighbouring eruptive rocks, or these sedimentary layers may have originated from 
tufaceous matters more or less closely resembling tho.se found at the present time on the 
bod of the Pacific. It is veiy^ probably in these conditions that zeolites occur in the 
argillaceous sehists at Andreasberg and Eule, in the lime.stones at Chappel, Fife, where 
a]>oj)hyllite with opal is observed fxWmg Stroi^honemas, and in the sandstones of the Upper 
Tcrtiar}' at Crevacuore. But whatever their position, or the nature of the rock in which 
they are fonned, these silicates always pre.sent characters indicating hydrochemical 
origin. It may also be stated, as the result of a comsiderable number of observations, 
