COAL MEASURES BENEATH ESSEX.
131
primaeval crust (a state of things perhaps not altogether unlike that
which exists in the larger planet Jupiter), that crust was much
thinner than the present lithosphere, or stony outer structure,
of the globe. Into this great question we cannot enter here.2
Now this thin crust at an early period began to depart from the
geometrical regularity of a spherical surface, and as the whole globe
contracted from further loss of heat, the crust had to bend and
throw itself into a series of slight flexures, rising in parts into broad
flat arches or domes (which initiated our present continents in out
line) and sinking in other parts into vast saucer-like hollows (out-
lining the ocean-basins of the globe). Further contraction neces-
sitated other movements of a more local nature, their localities being
determined by lines of weakness in the crust as it gradually thickened,
though not equally in all parts. Such movements have given
direction to most of the mountain-chains of the globe, some of which
have since been worn down by denudation to mere stumps, as is the
case with the Scottish and Welsh Highlands, the mountains of
Bohemia and Central Germany, and others that might be mentioned.
Along the early axes of elevation in many cases, further movements
(at the early stages feeble) have taken place, as the result of the
lateral thrust due to contraction of the globe as a whole, and in
their later stages have lifted into the sky such present lofty ranges
as the Alpine System, the Pyrenees, the Himalayas, the Andes.
Two general accompaniments of such axes of elevation must be
mentioned—(1) the fundamental crystalline rocks (the gneisses and
schists of the text-books) form everywhere the inner cores of these
elevated regions ; (2) such disturbances have given rise to innumer-
able foldings and faultings of the crust, producing lines and points
of weakness, through which enormous quantities of molten material
have escaped from the interior, giving us another class of crystalline
rocks known to the geologist as eruptive igneous rocks. These have
been simply passive accompaniments of elevation ; the notion (found
in the older text-books) that they were the active agents of elevation
may be relegated to the limbo of geological fiction. This has all
been discussed by myself and others elsewhere, but its discussion
here would be but of place, and involve scientific technicalities which
2 It is dealt with in my "Chemical and Physical Studies in the Metamorphism of Rocks"
(Longmans, 1889). The view there worked out has since been adopted by a leading American
geologist as furnishing the only clue to the observed relations of the Archaean Rocks of the
North American Continent and was warmly appreciated at the time by Professor Hermann
Credner, of Leipzig, whom I regard as one of the very foremost of European geologists. These
ideas are slowly working their way into the text-books published in this country.