in the case of the porphyritic granites of Galway, Shap Fell, and Syene in Egypt. But perhaps the most generally admired of the varieties of granite are those which, like the Peter head stone, are of a rich pink colour, arising from the predominance of rose-coloured felspar. There are indeed few rocks in this country which are superior to such in richness of colour, and suitability for ornamental purposes, especially when they are capa ble of being extracted in blocks of large size, and of receiving a high polish. Foliated Granite, or Gneiss. The minerals of which granite is composed are sometimes arranged in parallel layers or leaves, in which case the rock is said to be ‘ foliated/ and passes into gneiss. This structure is often observable over large tracts of country, as in Donegal and Galway, and is usually characteristic of those masses which have been formed by a process of metamorphism. The structure also is sometimes only apparent when the rock is viewed in its natural position, and often is persistent as regards general direction over large tracts of country. Graphic Granite. This variety seems to occur almost exclusively in veins, particularly those which traverse gneiss. 1 It contains but little mica; and the peculiarity of the structure to which it owes its name is due to the arrangement of the quartz and 1 McCulloch’s Gteol. Classification of Rocks, p. 335 (1821).
