Crosby.] 
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[November 2, 
and the latter to compact rocks. The term compact has, however, 
been so greatly misused that, to many, it will seem impossible 
that a rock can be at the same time compact and earthy ; but, 
properly viewed, this couplet stands on the same basis as fria- 
ble crystalline ; and the propriety of the latter none will dispute. 
This part of the subject may be summarized in the statement 
that the secondary textures have no existence apart from the 
primary textures ; and that, therefore, the texture of no rock can 
be fully described by the terms banded, porphyritic, vesicular, 
etc., but these must in every case be linked to the name of one of 
the primary textures. 
PART II. STRUCTURES OF ROCKS. 
An inspection of the standard works on geology shows that the 
treatment of the structures of rocks is quite as various as that of 
rock-textures. It is difficult to find two authors who take up the 
different kinds of structure in the same order ; and it seems to 
have occurred to none that a natural classification is a possibility. 
A natural classification is not so vital here as with textures to 
accurate description, but it does appear essential to comprehen- 
sive and philosophical views in this department of geology. 
We should distinguish primarily those structures developed in 
rocks at the same time the rocks themselves are made, from those 
which have been produced subsequently to the formation of the 
rocks. In other words, the most fundamental division possible 
among rock-structures is that separating the original from the 
subsequent structures. This is the basis of the classification, and, 
before proceeding further, a few words in demonstration of its 
naturalness will be in order. 
The original structures are of necessity produced by the same 
forces or agencies as the rocks in which they occur. Now it goes 
without saying that the great classes of rocks, as the stratified 
rocks, eruptive rocks and vein rocks, are formed by essentially 
distinct agencies ; and since dissimilar causes must produce unlike 
structures as well as unlike rocks, it follows that the original 
structures must be peculiar in every case to the great class or 
division of rocks in which they occur. A few examples will 
make this point clear. Stratification is the most important struc- 
