106.  . Mr. Dana on the Analogies between the 
structure corresponds with the constitution of the rock and pro- 
ceeds from its mineral composition, we shall see farther reason 
for rejecting this assumption. The general principle upon which 
this structure depends, appears to be simply this:—An igneous 
rock is in general more or less schistose or slaty, according to the 
cleavability of its constituent minerals. _ It is one of the general 
principles of crystallography, that when crystals of any mineral, 
form simultaneously, they tend to assume parallel positions. 
Faces of like cleavage lie in the same direction. No one can have 
glanced his eye over a druse of crystals, without being struck with 
the successive flashes of light that sparkle over the surface as its 
position is changed;.and if he has observed attentively, he has 
perceived that a similar face in each of the crystals reflects simul- 
taneously, and thus produces this beautiful effect. This is an 
instance of that parallelism in the position of crystals to which I 
have alluded. The same parallelism takes place in mineral aggre- 
gates, such as basalt or granite. Basalt is often described as hav- 
ing a regular cleavage and its columns as crystals. ‘There is no 
proper analogy between the forms assumed by mineral aggregates 
and crystals; for such mixed compounds cannot crystallize asa 
whole. Each constituent mineral of basalt or granite crystallizes 
independently, and one or the other, according to that which pre- 
dominates, impresses its cleavage upon the rock, or at least. gov= 
erns it to some extent in its fractures. The common cleavage of 
granite illustrates these facts. .The rock consists of quartz, feld- 
spar and mica—the first has no cleavage and the last yields in 
quantity to the feldspar, which is therefore the mineral upon which 
the cleavage of the rock depends. The unequal rectangular 
planes of fracture in granite rock known by every quarrymad, 
_ correspond therefore, as has been before suggested, with the cleav- 
ages of the contained feldspar. 
_If we examine the various igneous rocks with reference to. this 
, principle, we shall find them supporting it throughout.. Basalt 
consisting of feldspar and augite, and generally more or less chry- 
solite, is usually like granite, one of the uncleavable,igneous rocks, 
or possesses it but indistinctly. Either of these minerals may 
determine the lines of fracture producing the columnar structure. 
In New South Wales, my attention was directed to a bed of what 
was called mica slate, overlying. basalt. In hand-specimens it 
could hardly be. distinguished from a rusty decomposing mica 
