The SgÙrr of Eigg first attained to geological prominence in 1865, when Sir Archibald Geikie in his delightful account of the Scenery of Scotland offered a novel and altogether captivating theory to account for its origin [1]. According to Geikie, as all well know, the precipitous ridge of pitchstone, which culminates at its eastern extremity in the SgÙrr, is the inverse of an ancient valley sunk by a winding river in the basaltic plateau of the west. Before the development of this river channel, so the theory runs, the sources which had supplied the basalt lava of the plateau had already become extinct; but volcanic activity was not yet entirely banished from the region, and presently a great outpouring of acid lava, entering the valley, flowed for miles along its course, gradually choking it, perhaps even to the brim. The resulting pitchstone has stood the test of time much more securely than the neighbouring basalts, for while these latter have wasted to a level in general lower than that of the old valley floor, the pitchstone itself still remains in large measure unaffected, and thus furnishes a somewhat battered cast of the erstwhile hollow.
Petrografia das rochas ácidas da Formação Serra Geral na região do Rio Paranapanema - São Paulo