FOREIGN CORRESPONDENCE. 
849 
' The structure of the latter is observed to have become prismatic, 
lithoid, cellular, vitreous, or even marbled. When a sandstone has 
been thus modified, it has lost its red colour, which is replaced by 
■white, grey, green, or black tints ; the rock has become sonorous, and, 
under the hammer, breaks into splinters. When of a friable nature, the 
grains of quartz in the siliceous rocks have been cemented together by 
metamorphism, and even in the sandstones which present the most 
compact appearance the arenaceous structure is easily rendered evident 
by the action of an acid. When a prismatic structure has been taken 
by sandstones, the prisms are well-defined and perpendicular to the 
surface of contact with the trap-rock ; their section is small, but their 
length attains sometimes as much as two yards and more. Sandstone 
■which has become prismatic by the contact of basalt, contains a certain 
quantity of water, and when the former rock has taken a vitreous or 
cellular structure at the same time, it has undergone violent metamor- 
phism, but still contains water, and its density has been diminished. 
" The metamorphism experienced by argillaceous rocks from contact 
with traps," says M. Delesse, " is very difiicult to define, as the former 
contain almost all the elements that are found in the eruptive rock that 
has modified them. The proportions of these elements, then, alone 
can vary, hence chemical analysis can only show us the modifications 
these proportions have undergone. Moreover, as these rocks are very 
compact, minerals can only be formed in their cavities and fissures ; and 
it is easy to see that the latter do not differ from those formed in 
limestones and siliceous I'ocks under the same influence." 
The structure of argillaceous rocks, however, has undergone, by 
metamorphism, very great transformation, in a thousand different ways. 
An argillaceous rock may, however, sometimes be seen in immediate 
contact with traps, without having undergone the slightest change. 
On the other hand, the structure of the former is often observed to 
have become polyhedral, pseudo-regular, spheroidal, or even prismatic 
— the prisms being formed of hard clay, which has shrunk, but which 
contains as much water as the original unaltered clay. Generally 
speaking, argillaceous rocks that have been modified by upheaving 
traps, &c., have become hard and lithoid, or stony, and have lost their 
water and the carbonates they contained originally. By contact w ith 
trap-rocks in which zeolites are abundant these clay strata have often 
been changed to Pelagonite, being penetrated at the same time by 
different species of zeolites, carbonate of lime, silica, and the minerals 
peculiar to the Amygdaloidal rocks. They have likewise become 
cellular and transformed into spilite, especially if they be calcareous, 
in which case they lose, by metamorphism, the greater part of their 
carbonates. In these cavities are seen, also, the minerals peculiar to 
"Amygdaloids. 
Argillaceous rocks have often been changed into jasper by the meta- 
morphic action of certain traps. In this case they preserve, more or 
less, the marks of their original stratification, which are represented by 
parallel bands or veins. They have thus become hard, compact, and 
of bright and varied colours. 
Porcelanite-jasper is observed in contact with basalt-rocks, its cha- 
