parallel laminæ, the coarse and fine layers often alternating in regular bands about half-an-inch in thickness.1 Some of the sheets of felstone, 800 or 1,000 feet thick, are found, when followed along their strike, to split up and let in alternations of beds of ash and beds of limestone, showing that the greater uninterrupted masses were in reality formed by successive flows of molten rock at the bottom of the sea, and that, where each of these flows terminated or became thin, accumulations of ash or limestone took place on the sea-bottom in the intervals between the outpouring of one flow and that of the next, so as to cause these interstratifications. Pallas Hill is a conspicuous example of this occurrence. On the south side of this volcanic area there are five intrusive bosses of felspathic rock rising up through the lower limestone, and another boss at about the same distance on the the north side of the basin. These look like some of the volcanic foci from which the lavas were derived, the old roots, as it were, of the submarine lava flows, exposed to view by the denudation of the limestones and traps that once covered them. Other foci or intrusive masses are doubtless concealed beneath the existing beds of trap in the central parts of the basin. Professor Hull more recently has distinguished two epochs of eruption in the Limerick area, the first being an extrusion of felspathic lavas and ashes, while the second, which occurred after a considerable interval, gave birth to flows of augitic lava (basalt, etc.) The sect. (fig. 170), copied by permission from Prof. Hull's "Physical Geology of Ireland" illustrates the succession of materials erupted during the earlier outburst. 3. Scotland. The country which lies on either side of the Firth of Forth was anciently the scene of great volcanic activity. Arthur's Seat in Edinburgh and Largo Law in Fife are excellent instances of denuded volcanic piles; the numerous necks of Fifeshire have already been mentioned, and the behaviour of interbedded and intrusive sheets of 1 See the Explanations of Sheets 143, 144, 153, 154 of the Geological Survey of Ireland, and Jukes' "Manual of Geology," second edition, p. 325. 66 lava can be studied in the cliff sections along the coast.' Most of these eruptions took place towards the end of the Carboniferous period, but evidence of an earlier series of volcanic explosions are to be found in the ranges of the Sidlaw, Ochil, and Pentland Hills. "All these prominent ridges," says Sir A. Geikie,' are formed mainly of volcanic materials belonging to the period of the Lower Old Red Sandstone. Their general characters-beds of lava (porphyrite, etc.) with sheets of tuff and interstratified sandstones, shales, and conglomerates-leave no doubt whatever as to their having been truly erupted to the surface during that geological period. Basaltic Plateaux.-It has long been assumed that all erupted basaltic sheets were remnants of lava-flows which had once been connected with true volcanic cones. Baron Richthofen had indeed pointed out so long ago as 1868, that on the Pacific slope of the Rocky Mountains there is evidence of the emission of vast floods of lava without the formation of cones and craters, and he stated his belief that these eruptions had proceeded from great fissures out of which the lava had poured in a continuous stream. This theory was energetically opposed by Mr. Poulett Scrope, who demonstrated that the extensive plateaux of basalt in Auvergne were only separated from the cones with which they had originally been connected in consequence of the great erosion to which that country had been subsequently exposed. He argued that the apparent isolation of other plateaux was merely owing to the total destruction of the volcanoes from which they had been emitted (see Plate, fig. III). Subsequent observations, however, have tended to confirm the probability of Richthofen's views; the extent of these American lava-floods has been found to be enormous, and it is estimated that in the States of Oregon, Idaho, Nevada, and Northern California they cover an area of 200,000 square miles. The date of their emission is comparatively recent; they have been poured forth over the bottoms of the present valleys, and the only natural sec 1 Consult Sir A. Geikie on the "Volcanic Rocks of the Basin of the Forth, Trans. Roy. Soc. Edin.," xxix. p. 437. "Trans. Geol. Soc. Edin.," vol. ii. pt. iii. tions of them are those in the gorges or cañons which the present rivers have been able to erode out of the vast plain of basalt. Sir A. Geikie thus describes one of these enormous lava fields:- We had been riding for two days over fields of basalt among the valleys, and [at last] emerged from the mountains upon the great sea of black lava which seems to stretch illimitably westwards. It was as if the great plain had been filled with molten rock which had kept its level, and wound in and out along the bays and promontories of the mountain slopes as a sheet of water would have done. I looked round in vain for any central cone from which this great sea of basalt could have flowed. It assuredly had not come from the adjacent mountains, which consisted of older and very different lavas round the worn flanks of which the basalt had eddied. . . . I became convinced that all volcanic phenomena are not to be explained by the ordinary conception of volcanoes, but that there is another and grander type of volcanic action where the molten rock has risen in many fissures, and has welled forth so as to flood the lower ground with successive horizontal sheets of basalt." He suggests that the similar sheets and dykes of basalt which occur in the north of Ireland and west of Scotland, and the enormous dykes which are so frequent throughout Scotland and northern England, are all manifestations of a grand series of fissure eruptions comparable to those in North America, but belonging to an earlier geological period. In support of this conclusion he appeals to the wide extension and horizontality of the basalt sheets, the absence or paucity of interstratified tuffs, and finally thei apparent connection with a series of lava-filled fissures, some of which are no less than 200 miles distant from them. It should be mentioned, however, that the cogency of this evidence is disputed by Prof. Judd, and that contemporaneous volcanoes from which the lavas might have come certainly existed both in Scotland and Ireland. That portion of the great lava-flood which is isolated in the north-east of Ireland covers almost the whole of county 1 "Geological Sketches," by A. Geikie, p. 278. Antrim with a mass which is in some places 900 feet thick; it is 50 miles long by 30 wide, or about 1,200 square miles in area. The basalt mass consists of numerous sheets or flows, some of which are quite amorphous, either compact or amygdaloidal, while others are beautifully columnar; one of the columnar sheets dipping gradually into the sea on the north coast is known as the Giant's Causeway. Similar extensive plateaux of basalt occur in other parts of the world, in Abyssinia, western India, and Victoria, and probably mark the sites of some of the great fissureeruptions which have taken place at different times in the history of the world. In their recent résumé of the geology of India, Messrs. Blandford and Medlicott describe the persistent horizontality of the great basalt sheets which form the plateaux of the Deccan, the absence of any trace of associated volcanic cones, and the abundance of dykes which traverse the underlying platform of older rocks, where these emerge from beneath the basalt-covered area. THE CHAPTER XII. METAMORPHIC ROCKS. HE consideration of these rocks has been left till this stage because the peculiar structures which they exhibit could not be properly explained to the student until he had been made acquainted with the manner in which the unaltered rocks, both of aqueous and igneous origin, occur in the earth's crust, and had obtained some idea of the results caused by the intrusion of igneous among aqueous rocks, and by the earth-pressures to which both have been subjected from time to time. Metamorphic rocks may be broadly defined as those which, after their first consolidation, have been so altered that their internal structure and their physical characters are different from what they originally possessed. This change may be molecular, or atomic, or may be chiefly the result of mechanical stress or pressure; thus: (1) The molecular arrangement and physical character of the rock may be altered without much mineral change; for this kind of metamorphism Dr. Irving has proposed the name of metatropy. (2) The original mineral particles of the rock-mass may have been rearranged by mechanical stress; for this kind of change Dr. Irving has suggested the term metataxis.' (3) The mineral components of the rock may be recombined into minerals of different chemical composition; for this Prof. Bonney has suggested the excellent term metacrasis.' A simple case of metatropy is that of a limestone which 1 66 Metamorphism of Rocks," Longmans, 1889, p. 5. |