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Edinburgh Castle, enthroned on its trap-rock, once the centre of a volcano, is associated with the most stirring and important events in the history of Scotland; Stirling Castle rises on its trap-rock erupted by volcanic action above a vast plain, across which a hundred battles have swept; Dumbarton Castle, crowning its trappean promontory, has represented in its civil history the protracted periods of earthquake and eruption concerned in the formation of its site; while standing in solitude amid the stormy waters of the Firth of Forth, the Bass Rock, once a scene of fiery confusion, of roaring waves and heaving earthquakes, has formed alternately the prison where religious liberty has been strangled, and the fortress where patriotism has taken its last stand against the forces of the invader.
They abound in some trappean rocks and ancient lavas, where they fill up vesicular cavities and interstices in the substance of the rocks, but are rarely found in any quantity in recent lavas; in most cases they are to be regarded as secondary products formed by the action of water on the other constituents of the rocks.
I have already hinted at the close analogy in the forms of certain granitic and trappean veins; and it will be found that strata penetrated by Plutonic rocks have suffered changes very similar to those exhibited near the contact of volcanic dikes. Thus, in Glen Tilt, in Scotland, alternating strata of limestone and argillaceous schist come in contact with a mass of granite.
This, Sloggett explained, was the characteristic flame of both cyanogen and hydrocyanic acid vapour, which, being inflammable, may have become locally ignited in the passage over cities, and only burned in that limited and languid way on account of the ponderous volumes of carbonic anhydride with which they must, of course, be mixed: the dark empurpled colour was due to the presence of large quantities of the scoriae of the trappean rocks: basalts, green-stone, trachytes, and the various porphyries.
A similar disengagement of vapours, joined to the elastic force of the gases, which penetrate strata softened and raised up, appears sometimes to have given great extent to the caverns found in trachytes or trappean porphyries. They are lined with sulphur, and differ by the enormous size of their openings from those observed in volcanic tufas* in Italy, at Teneriffe, and in the Andes.
The leading varieties of the trappean rocks basalt, greenstone, trachyte, and the rest are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless masses protruding through or overlying them, or in horizontal sheets intercalated between strata.
The two principal families of trappean or volcanic rocks are the basalts and the trachytes, which differ chiefly from each other in the quantity of silica which they contain. The basaltic rocks are comparatively poor in silica, containing less than 50 per cent of that mineral, and none in a pure state or as free quartz, apart from the rest of the matrix.
Sandstone overlying coal; trap dykes; syenitic porphyry dykes; black vesicular trap, penetrating in thin veins the clay shale of the country, converting it into porcellanite, and partially crystallizing the coal. On this sandstone lie fossil palms, and coniferous trees converted into silica, as on a similar rock in Angola. IGNEOUS ROCKS. Trappean rocks, with hot fountain.
When we reflect on the changes above described in the strata near their contact with trap dikes, and consider how complete is the analogy or often identity in composition and structure of the rocks called trappean and the lavas of active volcanoes, it seems difficult at first to understand how so much doubt could have prevailed for half a century as to whether trap was of igneous or aqueous origin.
In order to establish these conclusions it was first pointed out that the calcareous rocks bordering the Gulf of Spezia, and abounding in Oolitic fossils, assume a texture like that of Carrara marble, in proportion as they are more and more invaded by certain trappean and Plutonic rocks, such as diorite, serpentine, and granite, occurring in the same country.
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