204 C. Lloyd Morgan — Geological Time. 



of tlie rate of accumulation of volcanic rocks, nor indeed is tHs 

 to be wondered at, since the formation of volcanic products is both 

 local in character and spasmodic in its mode of action. It will, 

 however, be useful to consider a few instances of volcanic accumu- 

 lation, which will show that in some cases the rate of this accu- 

 mulation may be comparatively rapid. Sir Charles Lyell, in 1828, 

 measured the thickness of volcanic ash which covered the town of 

 Pompeii, near the amphitheatre, and found it to be ten feet three 

 and half inches, which gives a rate of accumulation of rather more 

 than one-fifteenth of an inch per annum. Over Herculaneum, 

 which is nearer to Vesuvius, there has accumulated a thickness 

 of from 70 to 112 feet of material, giving since a.d. 79 a rate of 

 accumulation of some two-thirds of an inch per annum. After the 

 eruption of Coseguina in 1835, " eight leagues to the southward 

 of the crater, the ashes covered the ground to a depth of three 

 yards and a half," so that, if we suppose such an event to happen 

 once in 1800 years, the rate of accumulation of this material over 

 that spot would be one-tenth of an inch in a year. With regard 

 to lava, the well-known eruption of Skaptar Jocul in 1783 poured 

 forth two streams, of which one was fifty miles in length, with an 

 extreme breadth of some fifteen miles ; while the other was forty- 

 five miles in length, with an extreme breadth of seven miles, the 

 average depth of each being one hundred feet. If such an eruption, 

 or minor eruptions equalling it in the amount of material emitted, 

 took place once in every twelve hundred years, the average rate of 

 deposit would be one-tenth of an inch per annum, and this without 

 taking into account the immense volume of ash shot forth from 

 Skaptar Jocul during the same eruption of 1783. Beneath the 

 waters of a sea, in the neighbourhood of a frequently active volcano, 

 a deposit of volcanic ash may form with comparative rapidity, while 

 the rate of growth of the volcano itself would be still greater. The 

 highest marine clays on the flanks of Etna are some 1258 English 

 feet above the level of the Mediterranean, and within them are 

 contemporaneous basaltic products, "the most ancient monuments 

 of volcanic action within the region of Etna." The total height of 

 the mountain being 10,874 feet, we may take it that of this, 9616 

 feet are due to volcanic accumulations. Let us take 4500 feet as 

 the average height over a large area, say some 400 square miles. 

 Then, if the mountain began to accumulate, as Sir Charles Lyell 

 supposes, at the time of the Norwich Crag, and if this period may be 

 placed, on Mr. Croll's estimate, some 270,000 years ago, then the 

 rate of the accumulation of the volcanic products of Etna must 

 have averaged about one-fifth of an inch per annum. In the 

 absence of data of any certainty therefore, we may take -is of an 

 inch per annum as the rate of volcanic accumulation. 



Let us, now, take the thickness of the rocks at the liberal 

 figure of 100,000 feet. And let us suppose that of these 50,000 ft. 

 were deposited at the rate of x^ cr of an inch per annum, and of the 

 remaining rocks, 25,000 were laid down at the rate of -ro of an inch 

 per annum, and 25,000 were formed in delta or analogous deposits 



