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Show 356 JUNCTION OF GRANITE AND LIMESTONE. [Ch. XXV, analoO'ons to those which the contact of a fused mass might be 0 supposed to produce. No.88. Junction of granite und limestone in Glen Tilt. a, Granite. b, Limestone. c, Blue argillaceous schist. The above diagram from a sketch of Dr. Macculloch, re· presents the junction of the granite of Glen Tilt in PeJ·thshire, with a mass of stratified limestone and schist .. The ranite in this locality, often sends forth so many vems as ; 0 reti~ulate the limestone and schist, the veins diminishing towards their termination to the thickness of a leaf of paper or a thread. In some places fragments of granite appear entangled as it· were in the limestone, and are not visibly connected with any larger mass, while sometimes, on the other hand: a lump of the limestone is found in the midst ,~f .the gramte. 'l""he ordinary colour of the limestone of Glen 1~lt IS lead blue, and its texture larcre grained and highly crystalline; but where b • • t d it approximates to the granite, particularly where It IS penetra e't by the smaller veins, the crystalline texture disappears, and 1 Ch. XXV.] GRANITES OF DIFFERENT AGES, 357 assumes an appearance exactly resembling that of horn-stone. The associated argillaceous schist often passes into hornblende slate, where it approaches very near to the granite*. In the plutonic, as in the volcanic rocks, there is every gradation from a tortuous vein to the most regular form of a dike, such as we have described as intersecting the tuffs and lavas of Vesuvius and Etna. In these dikes of granite, which may be seen, among other places, on the southern flank of Mount Battoch, one of the Grampians, the opposite walls sometimes preserve an exact para11elism for a considerable distance. It is not uncommon for one set of granite veins to intersect another, and sometimes there are three sets, as in the environs of Heidelberg, where the granite of the right bank of the Rhine is seen to consist of three varieties differing in colour, grain, and various peculiarities of mineral composition. One of these, which is evidently the second in age, is seen to cut through an older granite, and another, still newe1·, traverses both the second and the :first. These phenomena were lately pointed out to me by Professor Leonhard at Heidelberg. In Shetland there are two kinds of granite. One of these, composed of hornblende, mica, felspar, and quartz, is of a dark colour, and is seen . underlying gneiss. 'l'he other is a red granite which penetrates the former everywhere in veins t. Granites of different ages.-It was formerly supposed that granite was the oldest of rocks, the mineral product of a particular period or state of the earth formed long antecedently to the introduction of organic beings into the planet. But it is now ascertained that this rock has been produced again and again, at successive eras, with the same characters, penetrating the stratified rocks in different regions, but not always associated with strata of the same age. Nor are organic remains always entirely wanting in the formations invaded by granite, although their absence is more usual. Many well authenticated exceptions to the rule are now established on the authority of * Macculloch, Geol. Trans., vol. iii. p. 259. 1' Macculloch, Syst. of Geol., vol. i. p. 58. |