GEOGNOSY AND GEOLOGY. 201 
cretaceous system. Granite is frequently met with that has been traversed 
by newer granite ; then if we find that the older of these granites is more 
recent than, for instance, the variegated sandstone, the newer must be still 
more recent. In many cases, however, it cannot be determined whether it 
be newer than the Muschelkalk, Keuper, Jura, or chalk, which are supported 
by the variegated sandstone. 
Where abnormal masses come in contact with normal, so that the latter 
are traversed by the former, changes are generally produced, as well with 
respect to the extensive as the intensive peculiarities of the latter. The 
changes of external character have reference to the position of the strata, 
which may be elevated, upheaved, displaced, broken, or even inverted ; 
those of internal character relate to alterations in the petrographical 
character of the rocks, as the chemical constitution and the condition of 
aggregation. Sometimes abnormal masses, in penetrating normal, take up 
a position between the strata of the latter, and thus acquire a pseudo- 
stratification which it may require considerable acuteness to detect. 
Masses often occur which can be referred neither to normal nor abnormal : 
they owe to the latter their origin, and have been stratified by water; such 
are various conglomerates, as basalt, trachyte, leucitophyr, and other conglo- 
merates, &c. In general, abnormal masses belong to the isonomic division : 
hey occasionally are heteronomic, in being accompanied by rocks of the 
litter character. They are always crystalline, and where this is not evident 
in the fresh fracture, the weathered surface will frequently exhibit it. A 
glissy texture is highly characteristic of an igneous origin; where this is 
noiexhibited, other phenomena may lead to the same conclusion. 
‘Ihe mineral substances composing the abnormal masses are principally 
silictes, or compounds of silicic acid. Among these may be mentioned 
feldsjar, mica, pyroxene, and amphibole. Pure silicic acid in the form of 
quart: is of rarer occurrence as an essential component, and that only in 
the oler masses. Oxydes of iron occupy a conspicuous place, these making 
their aypearance in the more recent abnormal rocks, in proportion as the 
silicic «id disappears. 
The vhole character of abnormal masses is opposed to their possession 
of organc remains. Haussman has distinguished three orders according to 
the relatire ages, as far as this can be ascertained. 
1. Puutonic. Rocks. 
Plutonic ncks are embraced within the region of the primary or bottom. 
and middle sries, and are the cause of many of the changes to which these 
have been sujected. The principal rock species are granite, syenite, eurite, 
and other porhyries; amphibolic rocks, especially greenstone ; pyroxenes, 
as euphotide, Cabase, trap, serpentine, &c. They are not found in definite 
succession, ancthe same species often belongs to different formations. 
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