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Show INTERMITTENT CHARACTER OF ERUPTIONS EXPLAINED. 141 hundreds of thousands and evea millions of years, should the same vent or cluster of vents yield so many different kinds of lava? So completely do the facts of volcanology antagonize the primordial character of lavas, that we seem driven to seek an opposite theory of their origin. These difficulties cease to be such and become normal phenomena when we take the postulate of local increments of temperature. The refusion of rocks becomes a slow and very gradual process. But when the melted rock is ready for issue, it does not follow that a steady stream of lava would keep flowing as long as the temperature continues to rise. We must now take into consideration the mechanism by which the expulsion is effected. This has already been suggested as the weight of overlying rocks crowding in upon the reservoir, and as these rocks are rigid relatively to small reservoirs, there is a limit to the smallness of the eruption. As the quantity of melted rock increases, this rigidity relatively diminishes until rupture takes place and all the lava hitherto accumulated is expelled. The overlying masses are then soldered up for a time, during which more lava is melted, and when the quantity is sufficient a second eruption occurs, and so the intermittent character is established and for a long period maintained. This assumption also explains the co-existence of vents at different levels, the presumption being that each vent derives its lavas from independent layers or maculae, and that several maculae or layers can successively find issue through the same vent when the magmas which they contain reach the eruptive condition. There is, however, one comprehensive or generalized fact connected with volcanoes which this assumption does not explain by itself, though it is not in any obvious respect inconsistent with it. This is the geographical distribution of volcanoes. It is well known that existing and recently extinct vents stand in the vicinity of the ocean and large bodies of inland water; a few exceptions, however, being known. But it has been repeatedly remarked that the postulated rise of temperature is asserted to be a proximate cause, itself requiring explanation by the production of some ulterior exciting cause. If we were able to find this ulterior cause, we should then know why volcanoes have their present distribution. It may be proper to remark here that this distribution would lead us to look for that cause in occur- |