But where, as probably happens in the large majority of compound dykes, there is a strongly marked difference between the respective bands of rock, we must either infer that two essentially different magmas co-existed in the volcanic reservoirs underneath, and were successively injected into the same fissures, or that a sufficient lapse of time occurred to permit a total renewal of the nature of the magma, and an uprise of this changed material into fissures which sometimes coincided with older dykes. If any interlocking of the crystals of the several bands of a compound dyke could be detected, we might suppose that the first-injected material had not become consolidated and cold before the uprise of the newer rock. But in general it would seem that so sharp a line of demarcation can be drawn between the two rocks as to indicate that their protrusion was due to two distinct and perhaps widely-separated volcanic paroxysms. Compound dykes of basic material occur not only among the ordinary straight north-westerly series, but also among the less regular gregarious dykes and veins, such as abundantly intersect the gabbro bosses. Moreover they are to be found among the youngest intrusions, for they traverse the masses of granophyre. Conspicuous examples of such late compound dykes are displayed along the cliffs of St. Kilda, as will be more particularly described in a later Chapter. These St. Kilda dykes often occupy not vertical fissures but parallel rents with a gentle inclination (see Figs. 367, 368). The Tertiary volcanic series of Scotland furnishes many examples of compound dykes of a much more complex character where parallel bands of some acid (granophyre, felsite, quartz-porphyry) or intermediate (andesite) rock is associated with others of the more usual basic material (dolerite, basalt, diabase). As the acid intrusions belong to a comparatively late part of the volcanic history, their modes of occurrence will be discussed in Chapters xlvi., xlvii. and xlviii. But no account of the general system of dykes would be complete without some reference to these compound examples, which will therefore be briefly described in the present section of this work. Early in this century some striking illustrations of the association of acid and more basic rocks within the same fissure were noticed by Jameson in the island of Arran. He described and figured instances at Tormore, on the west side of that island, where a group of pitchstones and "basalts" or andesites have been successively protruded into the same fissures in the (probably Permian) red sandstones of that district.[199] [Footnote 199: _Mineralogy of the Scottish Isles_, 1800.]