as lavas, we meet with veins and bosses which have been intruded into these rocks as well as into the associated sandstones. There can be little doubt that these intrusive masses in the Old Red Sandstone (quartz-porphyrites, and felstones) are of the same general date as the sheets of interbedded volcanic material in the same formation. In such bosses as Tinto, Quothquan, and other isolated hills in Lanarkshire, as well as in the Ochils and in Ayrshire, we probably see the true necks or lower portions of the volcanic funnels through which the lavas and tuffs were ejected."

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 confirmed 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 sections 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.

Dr. 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. Recent renewed examination of the basalt plateaux and associated dykes in the west of Scotland has assured me that this view of their origin and connection, which first suggested itself to my mind on the lava-plains of Idaho, furnishes the true key to their history."

Dr. Geikie 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, the want of any satisfactory evidence for the thickening and uprise of the basalt towards what might be supposed to be the vents of eruption, and finally their apparent connection with a series of lava-filled fissures, some of which are no less than 200 miles distant from them.

That portion of the great lava-flood which is isolated in the north-east of Ireland covers almost the whole of

1 "Geological Sketches," by A. Geikie, p. 278.

county 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.

CHAPTER XI.

PETROLOGY OF IGNEOUS ROCKS (continued).

Class II. Intrusive Rocks (Volcanic and Plutonic).

We have now to consider the intrusive masses of igneous rock which occur in the earth's crust, those portions which have been injected either into vertical cracks, or between the layers of stratified rocks, or have as it were eaten their way upward, till they now appear as irregular masses, displacing or replacing a certain amount of stratified rock.

In the last chapter we dealt exclusively with Volcanic rocks, and had no occasion to mention those to which the name of Plutonic is often applied (see p. 273); those, namely, which have cooled slowly under great pressure, and at great depths. Although this distinction is of little value in the lithological description of rocks, it becomes of some use when we have to deal with the petrological relations of those which occur in the form of intrusive masses. Some of these are referable to the Volcanic division, and others to the Plutonic. For our present purpose, therefore, in order to describe all such masses of igneous rock as are intrusive among other rocks, we may regard them as belonging to one of the three following series:

The first of these series consists of rocks that are essentially Volcanic; they are all close-grained, lava-form rocks

crystallized from a state of fusion (see p. 257), and only occur intrusively in the form of dykes, veins, and sheets.

The third series includes those rocks which are essentially Plutonic. They are all so coarse-grained that the term granitoid is sometimes applied to them collectively. They have crystallized from a state of fusion at great depths, and usually occur in the form of large irregular masses or bosses from which dykes and veins may or may not proceed.

The second series comprises rocks of an intermediate character, which sometimes occur in the massive form, and sometimes in the form of sheets or dykes.

The rocks which are placed on the same line form groups which have a natural connection with each other, so that we may expect to find rhyolitic lavas connected with dykes of quartz-felsite, and the latter proceeding from a mass of granite.

A. VOLCANIC DIVISION.

Dykes and Veins.-A dyke is a wall-like mass of igneous rock which has been intruded along the line of a more or less vertical crack or fissure. Dykes have already been mentioned in Part I., Chapter II., but those described in that connection were chiefly the smaller dykes which intersect the volcanic cone itself, and not the more deep-seated dykes which traverse the underlying rocks. When such dykes are exposed to view by subsequent denudation, the baked or partially fused condition of the rocks which form the sides of the lava-filled fissure clearly indicate the igneous character of the intruded material. The extent to which the adjoining rocks are thus metamorphosed varies from a few inches to several feet; sandstones are changed into quartzite, shale is indurated or vitrified into hornstone, limestones are made compact and crystalline.

The intersection of stratified rocks by dykes of igneous rock is illustrated in fig. 130, which represents vertical dykes of basalt traversing nearly horizontal sandstones in the Isle of Skye; some of these dykes have decomposed and crumbled away, leaving vertical gaps or trenches in place of the igneous rock which originally occupied them. The opposite coast of Ireland (county Antrim) is simi