602 
ELEMENTS OF GEOLOGY. 
has been since confirmed by the observations of three able 
geologists, Messrs. Ramsay, Harkness, and Geikie. The new¬ 
est of the series is a clay-slate, on which, along the southern 
borders of the Grampians, the Lower Old Red, containing 
Cephalaspis Lyell% Pterygotus Anglicus^ and Parka decipiens^ 
rests unconformably. 
Order of Succession in Metamorphic Rocks. —There is no 
universal and invariable order of superposition in metamor¬ 
phic rocks, although a particular arrangement may prevail 
throughout countries of great extent, for the same reason 
that it is traceable in those sedimentary formations from 
which crystalline strata are derived. Thus, for example, we 
have seen that in the Apennines, near Carrara, the descend¬ 
ing series, where it is metamorphic, consists of, 1st, saccha¬ 
rine marble; 2dly, talcose-schist; and 3dly, of quartz-rock 
and gneiss: where unaltered, of, 1st, fossiliferous limestone; 
2dly, shale; and 3dly, sandstone. 
But if we investigate different mountain chains, we find 
gneiss, mica-schist, hornblende-schist, chlorite-schist, hypo- 
gene limestone, and other rocks, succeeding each other, and 
alternating with each other in every possible order. It is, 
indeed, more common to meet with some variety of clay- 
slate forming the uppermost member of a metamorphic se¬ 
ries than any other rock; but this fact by no means implies, 
as some have imagined, that all clay-slates were formed at 
the close of an imaginary period when the deposition of the 
crystalline strata gave way to that of ordinary sedimentary 
deposits. Such clay-slates, in fact, are variable in composi¬ 
tion, and sometimes alternate with fossiliferous strata, so 
that they may be said to belong almost equally to the sedi¬ 
mentary and metamorphic order of rocks. It is probable 
that, had they been subjected to more intense plutonic ac¬ 
tion, they would have been transformed into hornblende- 
schist, foliated chlorite-schist, scaly talcose-schist, mica-schist, 
or other more pefectly crystalline rocks, such as are usually 
associated with gneiss. 
Uniformity of Mineral Character in Hypo gene Pocks, —It 
is true, as Humboldt has happily remarked, that when w^e 
pass to another hemisphere, we see new forms of animals and 
plants, and even new constellations in the heavens; but in 
the rocks we still recognize our old acquaintances—the same 
granite, the same gneiss, the same micaceous schist, quartz- 
rock, and the rest. There is certainly a great and striking 
general resemblance in the principal kinds of hypogene rocks 
in all countries, however different their ages ; but each of 
them, as we have seen, must be regarded as geological fami- 
