REPORT ON THE DEEP-SEA DEPOSITS. 
401 
Daubree has shown that zeolites are even in process of formation in the Roman bricks 
and concretes of the springs at Plombieres, and around the edges of other thermal wells. ^ 
But, as far as we know, they have never before been found in an isolated condition — as 
simple or twinned crystals, or free radiated aggregates — as we find them in the deposits 
of the Pacific and Indian Oceans. The deposits containing these zeolitic crystals present 
in these regions a totality of phenomena that appears never to have been realised on the 
same scale in the sedimentary formations of any geological period, unless, indeed, it be 
admitted that all traces of them have been effaced by posterior changes. 
Physical Characters . — On examining the deposits, from the regions in which these 
zeolitic crystals occur, under the higher powers of the microscope, there is seen, in the 
midst of mineral and argillaceous matters and volcanic debris, an infinity of small prisms 
of sharply cut form generally covered with a yellowish deposit. . These microliths appear 
to be as numerous in the clay as the little crystals of rutile in certain slates, for example. 
They are generally simple and isolated, though in some cases they form aggregates or are 
twinned ; there are also spherolithic groups in which several of these zeolitic crystals are 
entangled together so as to form crystalline globules of sufficient size to be distinguished 
by the naked eye, giving a certain grain to the deposit. We will describe first the 
isolated crystals of minute dimensions, which are carried away along with the argillaceous 
matters of the deposit in the process of decantation. These microliths are coated with 
a thin layer of hydrates of iron and manganese, which gives them, and in fact the whole 
deposit, a brown or fawn colour. Their form is better observed after treating them with 
very weak acid, which frees them more or less perfectly from the accidental coating sub- 
stances. Thus cleaned the smallest crystals are seen to be colourless or slightly milky ; 
a large number of micrometric measurements gives them a mean diameter of 0'027 mm. 
in length, and 0’005 mm. in breadth. They have a pronounced prismatic, very simple, 
form; the elongated faces, which may be taken for the faces of the prismatic zone, form 
between them a right angle. They are terminated at the two extremities by two faces 
resembling a dome, inclined the one to the other at an angle approaching 120°. It is rather 
diflBcult to see other faces clearly ; those just indicated are observed with certainty, but 
it may be that the smallest crystals of phillipsite, or at least certain of them, are ter- 
minated by four faces instead of two at each extremity. At the two ends of the crystals 
traces of two other faces which appear as dwarfed may be seen, but they are too ill-defined 
to allow of their existence being definitely made out. As a matter of fact, however, these 
four faces do exist in larger individuals, as will be presently pointed out. Their form 
indicates that they are single individuals of the monoclinic system presenting the faces 
0P(c), 00 Poo (6), ooP(m), elongated following the edge cjh (see PI. XXII. fig. 1), an elon- 
gation which determines the prismatic form of the crystals. The faces having the 
appearance of forming a dome are those of the prism {in ) ; the angle formed by the two 
^ A. Daubree, Etudes synthetiques de Geologie exp4rimentale, pp. 180 et seq., Paris, 1879. 
(deep-sea deposits chall. exp. — 1891.) 
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