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Updated: May 5, 2025


CONTACT METAMORPHISM. Rocks beneath a lava flow or in contact with igneous intrusions are found to be metamorphosed to various degrees by the heat of the cooling mass. The adjacent strata may be changed only in color, hardness, and texture. Thus, next to a dike, bituminous coal may be baked to coke or anthracite, and chalk and limestone to crystalline marble.

A core of old red sandstone, which occasionally crops out at the surface, and through which in one spot, near Downhead, a vein of igneous rock has forced its way, is thickly coated with a crust of mountain limestone. The once superincumbent coal-measures are huddled together on one side in a confused heap near Radstock, and on the other are probably buried beneath the Glastonbury marshes.

That at this time, however, they had lost the higher truths involved in the conception of this Deity, is evident. They had become worshippers of fire, or of that subtle igneous fluid residing in fire which they believed to be creative force.

The proximity of heated masses of rock has promoted chemical action in the same way as do the Bunsen burners or the sand baths in the laboratory; but no case has yet come under my observation where it was demonstrable that the filling of a fissure vein had been due to secretion from igneous or sedimentary wall rocks.

Everywhere on the site and in the neighbourhood of Rome striking indications of ancient volcanoes abound. The whole region is as certainly of igneous origin, and was the centre of as violent fiery action, as the vicinity of Naples.

We have as yet no clew to the laws by which this distribution of volcanic elements in the formation of the earth is regulated; but there is found to be a difference in the crystals of the Plutonic rocks belonging to different ages, which, when fully understood may enable us to determine the age of any Plutonic rock by its mode of crystallization; so that the mineralogist will as readily tell you by its crystals whether a bit of stone of igneous origin belongs to this or that period of the world's history, as the palæontologist will tell you by its fossils whether a piece of rock of aqueous origin belongs to the Silurian or Devonian or Carboniferous deposits.

But the geological structure of the Pentland Hills, near Edinburgh, shows that igneous rocks were there formed during the newer part of the Devonian or "Old Red" period. These hills are 1900 feet high above the sea, and consist of conglomerates and sandstones of Upper Devonian age, resting on the inclined edges of grits and slates of Lower Devonian and Upper Silurian date.

The greater portion of these igneous rocks appear to have originated during the Upper Miocene and Pliocene periods; and extinct quadrupeds of those eras, belonging to the genera Mastodon, Rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.

Agriculture by the hand, then, and absolute refusal or banishment of unnecessary igneous force, are the first conditions of a school of art in any country.

But to admit the "Plutonic" origin of such widespread formations was practically to abandon the Neptunian hypothesis. So gradually the Huttonian explanation of the origin of granites and other "igneous" rocks, whether massed or in veins, came to be accepted. Most geologists then came to think of the earth as a molten mass, on which the crust rests as a mere film.

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