
CHAPTER Ti 
ROCKS 
Classification :—Crystalline Igneous Rocks—their general characters. 
Chief Minerals of Igneous Rocks. Primary and Secondary Minerals. 
Law of Mineral Combination. Groups of Igneous Rocks :—Rocks 
with Dominant Alkali Felspar ; Rocks with Dominant Soda-Lime 
Felspar; Rocks with Felspathoids in place of Felspars; Rocks 
without Felspars or Felspathoids ; Pyroclastic Rocks. 
THE’ term “rock,” ‘as used) by “he vccolosier means any 
mass or aggregate of one or more kinds of mineral or of 
organic matter, whether hard and consolidated or soft and 
incoherent, which owes its origin to the operation of natural 
causes. Thus granite, basalt, limestone, clay, sand, silt, and 
peat, are all equally termed rocks. 
Speaking generally, we may say that the unconsolidated 
rocks occupy for the most part a superficial position—over- 
spreading and concealing the consolidated rocks of which the 
earth’s crust is chiefly composed. There are many exceptions 
to this rule, however. Sometimes, for example, unconsolidated 
materials occur at considerable depths from the surface, buried 
under masses of hard rock. Nor is the relative age of a rock 
always indicated by the degree of its consolidation. Many 
incoherent rocks are of great geological antiquity; while, on 
the other hand, some rocks of quite recent age are never- 
theless as hard and resistant as the oldest. 
Classification of Rocks.—The rocks of which the earth’s 
crust is constructed are very diverse in character and origin. 
Some owe their origin to eruptive and volcanic forces ; others 
are obviously composed of materials which have been derived 
from the disintegration of pre-existing rock-masses; while 
yet others have undergone certain more or less fundamental 
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