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Updated: June 20, 2025
The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate.
Their actual elevation above the sea may be referred to the same causes to which we have attributed the upheaval of marine strata, even to the summits of some mountain chains. Difficulty in ascertaining the precise Age of a Plutonic Rock. Test of Age by Relative Position. Test by Intrusion and Alteration. Test by Mineral Composition. Test by included Fragments.
A considerable lapse of time must intervene between the formation of Plutonic and metamorphic rocks in the nether regions and their emergence at the surface.
Here I began to realize the universally igneous origin of Hawaii, as I had not done among the finely disintegrated lava of Hilo. From the hard black rocks which border the sea, to the loftiest mountain dome or peak, every stone, atom of dust, and foot of fruitful or barren soil bears the Plutonic mark.
It may be traced for many miles, passing through the amygdaloidal and other traps of the hill called Norman's Law in that parish. In its course it affords a good exemplification of the passage from the trappean into the Plutonic, or highly crystalline texture.
It is certainly in the Alps of Switzerland and Savoy, more than in any other district in Europe, that the geologist is prepared to meet with the signs of an intense development of Plutonic action; for here strata thousands of feet thick have been bent, folded, and overturned, and marine secondary formations of a comparatively modern date, such as the Oolitic and Cretaceous, have been upheaved to the height of 12,000, and some Eocene strata to elevations of 10,000 feet above the level of the sea; and even deposits of the Miocene era have been raised 4000 or 5000 feet, so as to rival in height the loftiest mountains in Great Britain.
If the foregoing views of the origin of trap-dikes in widely extended granitic regions far from rocks of any other formation, be admitted as probable, we may further admit, in the case of a great body of plutonic rock, being impelled by repeated movements into the axis of a mountain-chain, that its more liquid constituent parts might drain into deep and unseen abysses; afterwards, perhaps, to be brought to the surface under the form, either of injected masses of greenstone and augitic porphyry, or of basaltic eruptions.
In going from a district of the Old Red Sandstone to where I walk upon the old Plutonic Rock, not fifty miles distant, I miss in the woods, the veery, the hermit thrush, the chestnut-sided warbler, the blue-backed warbler, the green-backed warbler, the black and yellow warbler, and many others, and find in their stead the wood thrush, the chewink, the redstart, the yellow-throat, the yellow-breasted flycatcher, the white-eyed flycatcher, the quail, and the turtle dove.
Unaltered fossiliferous strata of every age are met with reposing immediately on Plutonic rocks; as at Christiania, in Norway, where the Post-pliocene deposits rest on granite; in Auvergne, where the fresh-water Miocene strata, and at Heidelberg, on the Rhine, where the New Red sandstone occupy a similar place.
It was also felt that some general term was indispensable, because these rocks, although very diversified in form and composition, evidently belonged to one group, distinguishable from the Plutonic as well as from the non-volcanic fossiliferous rocks. By degrees familiarity with the products of active volcanoes convinced geologists more and more that they were identical with the trappean rocks.
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