220 J. LOGAN LOBLEY, F.G.S., F.R.G.S., ON VOLCANIC ACTION 



coast-line to a very moderate distance, as in the neighbourhood 

 of Eorne. 



Steam is a most abundant and sea-salt a common product of 

 explosive eruptions, and some volcanic tufas consist largely of 

 marine Diatomacea as in Patagonia. 



Enormous flows of lava have been poured on to the surface 

 without explosive effects, and there are vast beds of lava-rock 

 not associated with volcanic scori?e or cones, as in Antrim, 

 Abyssinia, and Idaho ; and Plutonic igneous rock-masses and 

 dykes have, been also formed without explosive effects. 



Volcanoes quite dormant for many centuries have sometimes 

 commenced a new epoch of activity, as Vesuvius in a.d. 79 ; 

 and sometimes after an eruption have relapsed into complete 

 quiescence, as Epomeo in 1302. 



Pressure caused by shrinkage of the earth's crust would be 

 tangential, not vertical, and the heat produced by the crushing 

 pressure of rocks is not localized at the points of contact, but 

 disseminated through the rock-masses synchronously with 

 production. 



2,000,000 tons pressure are removed from every square mile 

 of the earth's surface when the barometer falls two inches, and 

 observations of the activity of Stromboli and Vesuvius seem to 

 indicate an approximation to periodicity of eruptive energy 

 coincident with (1) autumn and winter; (2) the lunar syzygies ; 

 and (3) with hygrometric atmospheric conditions. 



Antecedent to eruptions, earthquakes, or earth-tremors, occur,, 

 especially, and more violently, previous to the opening of new 

 vents, as at Jorullo and Monte Nuovo, and after long dormancy, 

 as before the first historic eruption of Vesuvius. 



Though great volcanic activity may occur at particular 

 periods, yet no sympathy between the eruptive energy at two 

 well separated vents has been with certainty found to exist. 



This statement of governing facts to be kept in view when an 

 endeavour is made to explain the cause of volcanic action, 

 though brief and incomplete, is perhaps sufficiently comprehensive 

 to prevent a too ready acceptance of inadequate hypotheses, 

 while it may serve to show as well the difficulty as the highly 

 interesting character of the subject. So difficult, indeed, does 

 it appear to the authors of geological works, that for the most 

 part they content themselves with a description of phenomena 

 and a statement of some hypotheses, while refraining from 

 giving gny explicit opinion of their own. 



