GEOGNOSY AND GEOLOGY. 127 
abnormal. We can estimate the age of one abnormal mass only in reference 
to another or to a normal. That phenomenon of interpolation is of not 
unfrequent occurrence, being often seen in some basalts. An instance of 
this is seen in the Electorate of Hesse, near Eschwege, where a basalt has 
pierced through the variegated sandstone, and overlies it. This relation 
has been revealed by an extensive stone-quarry which affords an excellent 
view of the whole circumstance. The sandstone has been discolored in the 
vicinity of the basalt, and melted with it at the immediate surface of 
contact. The basalt is thus newer than the variegated sandstone, or the 
abnormal younger than the normal. The same is also the case in the 
Meissner mountains in Hesse, where basalt has broken through the tertiary 
brown coal, and poured up over the top. The penetration of abnormal 
masses by others also abnormal, is likewise of frequent occurrence. Thus, 
in the vicinity of Heidelberg, granite may be seen which is traversed by 
veins of granite of entirely different petrographical character, of another 
color and other grain. The granite traversed must necessarily be the - 
older of the two. In volcanic regions, also, we frequently see trachytic 
rocks traversed by basaltic, or older lavas by younger, as shown in 
pl. 43, fig. 11. The dark portion is an older abnormal rock, pierced by a 
younger (the verticaily lined portion), and together with the dotted normal 
elevated by it. The alterations effected by abnormal masses in the 
stratification of normal cannot readily be mistaken. By the elevation of 
the former the strata of the latter have been upheaved, broken, and 
sometimes entirely inverted. PJ. 43, fig. 15, is a profile of a portion of the 
Hartz mountains, in which @ represents the Brocken, consisting of granite, 
which has upheaved and pierced through the normal transition slate, the 
individual strata succeeding, each in precisely the same relative order, and 
of the same size on each side. In this instance, as in many others, it is 
impossible to overlook the agency of the granite. By means of such 
elevations older strata are brought to view, and man enabled to ascertain 
facts with reference to subjacent beds, which must otherwise have remained 
buried in perpetual obscurity. Did the different layers envelope the earth 
with the regularity of the coats of an onion, it might be possible to pierce 
through one, or at most two of these, by mining operations, and no more. 
In the present condition of things, however, we find an entire succession of 
rocks revealed, with a limited extent of surface, which otherwise, at depths 
of entire miles, would have been beyond our ken. 
13. Formations, Groups, and Systems. 
If we consider the different rock beds, with respect to their constitution, 
we shall soon find that the most important are represented by only a few 
species of rocks ; limestone, sandstone, clay, and the marls, are those which 
occur most frequently. Nevertheless, they exhibit such decided characters 
in their different relations, as to render it impossible to mistake them in 
certain groups, and to fail of coming to the conclusion that they were 
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