44 
AMERICAN GEOLOGY. 
§ 37. Structure of the massive jryrocrystalline rocks . The 
peculiar mode in which these rocks became consolidated fur- 
nishes a clue to their essential structure. This is crystalline. 
V' They are not only composed of crystallized minerals, but they 
are crystalline in the mass. This statement is sustained by the 
fact that, in the quarry where large masses are raised, they 
split readily in certain directions: it is in fact a cleavage simi¬ 
lar in form to that of a simple mineral. These directions or 
joints of cleavage are developed by the disintegration of the 
rock by atmospheric causes, the action being always more per¬ 
ceptible in the direction of the cleavage planes; they appear 
to separate spontaneously, and to extend deeply into the rock. 
The rock in this condition shows all the directions in which 
it may be split. The annexed cut (fig. 1) illustrates the 
Fig. 1. 
appearance of a mass undergoing the changes alluded to, and 
by which it is separated into angular parts. This result is not 
to be regarded as a lamination, inasmuch as lamination is the 
result of the arrangement of different minerals in parallel planes; 
and the ready splitting in the direction of those planes is due 
to the diminished cohesion between tw T o different minerals in 
part, and in part, also, to diminished cohesion which always 
exists between the broad planes of crystals. The separation 
of the folia of mica or talc through their broader planes illus¬ 
trates this fact 
