1867.] Sir Charles Lyell and Modern Geology. 11 



avoid purely lacustrine and marine strata, although they often lie 

 immediately contiguous ? "* Secondly, it is in accordance with all 

 analogy to expect that if these great volcanic cones were upheaved 

 after the ejection of the matter composing them, their sides would 

 he fractured and the volcanic strata shattered and disturbed in a 

 considerable degree. But the reverse is the case, for of all isolated 

 hills volcanic cones are the most symmetrical in form, and regular 

 in the arrangement of their constituent materials. 



Of late years it has been asserted that volcanoes could not have 

 been formed by " eruption," because solid lava could not consolidate 

 on a slope greater than three degrees, nor vesicular lava on a 

 greater inclination than five degrees. But Sir Charles Lyell 

 proved! that this is an error as to a matter of fact. He showed 

 that several of the lavas of Etna of known date have formed 

 continuous beds of compact stone on slopes of 15, 36, and 38 

 degrees, and in one instance (the lava of 1852) of 40 degrees. 

 Other volcanic cones, such as the island of Palma, yielded similar 

 evidence, so that this objection to the " eruption " theory has been 

 fully answered. The objections to the "elevation" theory have 

 not ; they rest on a wider basis, so they probably never will. The 

 form of a volcanic cone is, moreover, precisely that which would be 

 produced by the falling of materials thrown vertically into the air 

 from a central vent. 



Sir Charles Lyell has naturally watched with great interest the 

 recent discussions on subaerial phenomena, more especially those on 

 the mode of formation of lake basins and on the origin of valleys 

 and the denudation of the Weald. His latest published examination 

 of these questions is contained in the sixth edition of his ' Elements 

 of Geology,' but they will probably be more fully discussed in the 

 forthcoming tenth edition of the ' Principles.' 



In the first edition of the latter work Sir Charles Lyell taught 

 that the Wealden area had been denuded by the sea, to which agent 

 he also ascribed the formation of the chalk escarpments ; but he 

 referred the formation of the transverse valleys to the action of 

 rivers running along lines of fracture. Professor Kamsay and 

 others have recently contended that "rain and rivers" and other 

 subaerial agents have produced all the surface-features, not only of 

 the Wealden region, but also of the whole terrestrial surface of the 

 globe, excepting of course volcanic cones and craters. In the 

 opinion of the advocates of this theory the sea has planed off 

 the surface of the land as it emerged, and this form has been termed 

 by Professor Bamsay the "plane of marine denudation." All the 

 existing physical features have been since produced by subaerial 

 erosion. This theory appears to go as much too far in one direction 



* ' Principles,' first edit., vol. i., p. 387. 

 f ' Philosophical Transactions,' 1858. 



