FOREIGN CORRESPONDENCE. 



349 



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 difficult 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 diff'er from those formed in 

 limestones and siliceous rocks under the same influence." 



The structure of argillaceous rocks, however, has undergone, by 

 metamorphism, very great transformation, in a thousand diff'erent 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 with 

 trap-rocks in which zeolites are abundant these clay strata have often 

 been changed to Pelagonitey 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 janper 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 m observed in contact with basalt-rocks, its cha- 



