88 - GEOGNOSY AND GEOLOGY. 
however, not the only means which has enabled us to become acquainted 
with the lower regions of the earth’s crust: we shall see subsequently, how 
certain conditions of stratification reveal to us structures and relations at 
depths which it might be impossible for us, otherwise, to ascertain. Just 
as geogeny requires geognosy as a necessary foundation, so does geognosy 
require mineralogy; since it is aggregations of simple minerals, either 
separately or in combination, that constitute the earth’s crust. It is not 
all mineral substances that are prominent in this respect, by far the greater 
number are not brought into view; it is proportionably very few that 
present themselves as important constituents of rocks. Of these few, some 
constitute entire formations singly ; others form large masses, only in 
combination with each other, or with the preceding. These different 
combinations, as well as the simpler mineral forms, all constitute a whole, 
geognostically speaking, to which we give the name of rocks. It is thus 
evident, that a mineral may occur as a rock, but that every rock must not 
necessarily be a simple mineral. Just asa mineral is compounded of simple 
elements, these being combined according to the rules of chemical affinity, 
so minerals may be compounded into rocks; the force influencing them, 
however, is not chemical affinity, but cohesion. Furthermore, as the 
ingredients of mineral bodies stand, to a certain extent, in necessary 
combinations, this may only be incidental in regard to the constituents of 
rocks. These rocks may be considered in respect to their mineralogical 
composition, or with regard to the relation in which they stand to each 
rother. The first brings them under the head of Petrography or Lithology, 
the latter under that of Oreography. Petrography treats of rocks in the 
minute, oreography in the great, or as constituting formations. The former 
bears somewhat the same relation to the latter, that mineralogy does to 
geognosy: the study of petrography must therefore precede that of 
oreography, in a philosophical examination of the entire subject. 
From what has already been said, it follows that rocks may be divided 
into simple or homogeneous, and mixed or heterogeneous; yet, however 
distinct the two ideas may be, it is sometimes difficult to say, to which 
kind a certain rock must be referred. The. compact varieties of the 
heterogeneous rocks sometimes present so intimate a mixture of ingredients, 
that at first sight they may not be seen in their true characters. The 
homogeneous are connected with the heterogeneous, by the most insensible 
transitions; as the heterogeneous are with each other. Thus what 
petrography loses in respect to the elementary variety of her forms, she 
more than makes up by the infinite diversity of their combinations. 
There is no special difficulty in determining rocks when they present 
themselves in their characteristic forms; the proper appreciation of many 
transitions of one rock into another, can, however, in many cases, only be 
effected by means of the help afforded us by Oreography. It is, never- 
theless, of the highest interest and importance to geologists to have an 
accurate knowledge of such transition forms, as they sometimes reveal to us 
very interesting affinities, or at least analogies between different formations. 
As before remarked, only a few minerals occupy. a prominent position in 
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