Report of a lecture entitled 



THE EVOLUTION OF IGNEOUS ROCKS. 



By blR J E THRO J. H. TEALL, m.a., d.sc, ll.d., f.r.s. 



President of the Bournemouth Natural Science Society. 

 (Delivered before the Geological Section, May 11, 1918.) 



The President began by pointing out that although igneous 

 rocks differ widely in composition the differences are such as 

 to suggest a sort of blood relationship or consanguinity ; and that 

 the idea had arisen that the rocks of special districts, and even 

 igneous rocks as a whole, have a common origin. He proposed 

 to consider the subject with reference to two great groups — the 

 andesites and the basalts — which, taken together, far exceed all 

 other rocks that have reached the earth's surface from below. 

 He would deal first with andesites, not because they were the 

 most important, but because they illustrated in the simplest 

 manner a principle of great significance in relat'on to the origin 

 of petrographical species. 



In 1902 a great eruption of Mount Pelee, an andesitic vol- 

 cano in the island of Martinique, took place, resulting in the 

 destruction of St. Pierre. Three kinds of solid material were 

 formed — ashes, bombs, and a mass of rock which first rose as a 

 dome in the centre of the old crater in a semi-molten condition. 

 A few months after the first eruption, the top of the dome 

 opened and some nearly solid rock was squeezed out as one 

 may squeeze oil paint from a metallic capsule. It was 

 sufficiently solid on the exterior to hold together, and so rose 

 vertically, forming a, "spine." In all three kinds of solid material 

 crystals occurred, and they were alike in every respect, but the 

 matter with which they were associated differed in the different 

 kinds. In the ashes it consisted of minute fragments of glass; 

 in the bombs and in the rocks of the dome and. spine it varied 

 from a glass to a mass almost entirely composed of minute crys- 

 tals, thus proving- that the larger crystals had been developed in 

 the magma before eruption, and that the glass and smaller crys- 

 tals, which had formed during or after eruption, represented the 

 mother-liquor left after the separation of the larger crystals. 

 Very little lava of the ordinary kind was formed. The magma, 

 with its included crystals, was mostly blown into the air by the 

 violent escape of gases and scattered far and wide. But large 

 volumes of precisely similar material, had flowed as lava, both 

 from ancient and modern andesitic volcanoes in various parts of 

 the world ; as for example, in the Cheviot district in Old Red 

 Sandstone times. 



The speaker then described the chemical composition and 

 microscopical structure of a lava from the Cheviot Hills which 

 agreed in composition with the material erupted by Mount Pelee, 



