Parr 1] THE TERRESTRIAL CRUST. 43 
must necessarily increase the specific gravity of the interior, as will 
be alluded to further on. 
§2. The Crust.—It was formerly a prevalent belief that the 
exterior and interior of the globe differed from each other to such an 
extent that, while the outer parts were cool and solid, the vastly more 
enozmous inner part being intensely hot was more or less completely 
fluid. Hence the term “crust” was applied to the external rind in 
the usual sense of that word. This crust was variously computed to be - 
ten, fifteen, twenty, or more miles in thickness. In the accompanying 
diagram (Fig. 4), for example, the thick line forming the circle 
represents a relative thickness of 100 miles. There are so many 
proofs of enormous and wide-spread corrugation of the materials of the 
earth’s outer layers, and such abundant traces of former volcanic action, 
that geologists have naturally regarded the doctrine of a thin crust over 
a liquid interior as necessary for the explanation of a large class of 
terrestrial phenomena. For reasons which will be afterwards given, 
however, this doctrine has been opposed by eminent physicists and 

Fic. 4.—Suprosep Crust or THE Earrs, 100 Mites THIcK. ° 
is now abandoned by most geologists. Nevertheless the term 
“erust”’ continues to be used as a convenient word to denote those 
cool, upper, or outer layers of the earth’s mass, in the structure and 
history of which, as the only portions of the planet accessible to 
_ human observation, lie the chief materials of geological investigation. 
The chemical and mineral constitution of the crust is fully discussed 
in later pages. 
§ 3. The Interior or Nucleus.—Though the mere outside skin of 
our planet is all with which direct acquaintance can be expected, the 
irregular distribution of materials beneath the crust may be inferred 
from the present distribution of land and water, and the observed 
differences in the amount of deflection of the plumb-line near the sea 
and near mountain-chains. The fact that the southern hemisphere 
