i66 



NATURE 



Yjfjine 27, 1872 



mcnced nearly all round an island, keep building up as 

 the island goes down till they have formed a ring of coral. 

 The accompanying ideal section across such an island 

 enables one to understand the mode of growth. A 

 channel is kept open through one side, probably at first 

 by the stream, which drains the island, and carries down 

 mud and fresh water, and afterwards by the scour of the 

 tide. Whenever an area covered by such islands is up- 

 heaved, and tlie reefs lifted up above the breakers, or the 



A The Ivland : /', c highest points of the encircling reef between which and 

 tlie coast is seen a space occupied by still water. 



waves and wind have heaped up broken coral rock and 

 shell around, the surface soon gets weathered, and forms 

 a soil on which plants and animals settle and live. Some- 

 times the top of the island around which the coral was 

 built is still seen ; sometimes it has disappeared alto- 

 gether beneath the sea. We subjoin a sketch of one of 

 these circular reefs. 



But besides such indirect evidence of gradual change 

 of level, it is a matter of observation that as an accom- 

 paniment of volcanic action we frequently have sudden 

 movements of small extent. For instance, in the de- 

 structive earthquake which visited Chili in 1822, the 

 coast was raised from 2 ft. to 4 ft., while farther inland 

 the rise was estimated at from 5 ft. to 7 ft., and off the 

 port of Penco, if the reports of the inhabitants are to be 

 believed, there was a rise of 24 ft. during the single earth- 

 quake of 1751. In New Zealand, during the earthquake 



of 1855, a fault 10 miles long, with a displacement of 9 ft., 

 was produced. 



Supposing an elevation of 7ft. occurred only once every 

 century, it would require less than 150,000 years to form 

 a chain as high as the Pyrenees, and if repeated three 

 times in a century would be sufficient to account for the 

 Andes in the same time. 



" It may be instructive," says Sir Charles, " to consider 

 these results in connection with others already obtained 

 from a different source, and to compare the working of 

 two antagonistic forces — the levelling power of running 

 water, and the expansive energy of subterranean heat. 

 How long, it may be asked, would the Ganges require 

 .... to transport to the sea a quantity of solid matter 



equal to that which may have been added to the land by 

 the Chilian earthquake? The discharge of mud in one 

 year by the Ganges at its mouth was estimated at 

 20,000,000,000 cubic feet. According to that estimate it 

 would require about four centuries before the river could 

 bear down from the continent into the sea a mass equal 

 to that gained by the Chilian earthquake " (p. 97). 



In volcanic districts especially we may expect evidence 

 of recent upheaval and depression, and so we often have 

 marine beds forming the base of a volcano, or submerged 

 volcanos, whose leading features seem to be due to sub- 

 aerial action. We may, for instance, mention the case of 

 Etna, and refer our readers to the interesting line of 

 reasoning by which our author works out the Iiistory of 

 that mountain, showing that it was fortiied by degrees, of 

 matter heaped up upon marine beds of comparatively re- 

 cent age, which have now been lifted up to a considerable 

 height above the sea, and further proves that at one time 

 there were two principal craters from which matter was 

 ejected, but that now, owing to subsequent explosions and 

 denudation, an enormous valley occupies what was the 

 top of the mountain. 



As an example of a submerged volcano we may mention 

 Santorin, with regard to which Sir Charles Lyell says : — 



" We may conceive, therefore, if at some former time 

 the whole mass of Santorin stood at a higher level by 

 1,200 feet, that this single ravine or n.arrow valley, now 

 forming the northern entrance, was the passage by which 

 the sea entered a circular bay. But at a still earlier 

 period, when the ancient volcanic cone — of which the 

 outer islands are the remains — was still more elevated 

 above the level of the sea, there may have been a deep 

 valley of subaerial erosion cut by the principal river which 

 then drained Santorin, which may have consisted of one 

 lofty volcanic cone, afterwards truncated by a paroxysmal 

 explosion such as we have already spoken of in the case 

 of Galongoon" (p. 72). 



We subjoin Sir Charles Lyell's sketch (Fig. 3), which 

 it will be interesting to compare with that of the unsub- 

 merged summit of Etna. 



We select also his ideal section across Barren Island 

 (Fig. 4), to help to realise its manner of formation. 



It may be worth calling attention to the similarity be- 

 tween the submerged crater, with its deep channel leading 

 into it on one side, and the coral Atoll of which we have 

 given figures above (Figs, i and 2). Nature has many 

 ways of arriving at apparently analogous results ; but close 

 examination shows how varied are her methods. 



Sir Charles also points out that, in the quantity of mat- 

 ter ejected, modern eruptions will bear comparison with 

 any we know of in ancient times. In order to help us to 

 realise the enormous volume of the lava poured out from 

 Skaptar Jokul in 1783, he considers "how striking a 

 feature" the two streams of lava then poured out "would 

 now form in the geology of England, had they been 

 poured out on llie bottom of the sea after the deposition 

 and before the elevation of our secondary and tertiary rocks." 

 From one we should have a mass too ft. thijk and 10 to 

 I 5 miles broad on the oolitic hills overlooking the vale of 

 Gloucester. It would be traced for a distance of about 

 90 miles to the neighbourhoodof London, where, "crown- 

 ing the highest sands of Higligate and Hampstead, we 

 might behold some remnants of the current some 500 ft. 

 or hoo ft. in thickness, causing those hills to rival or even 

 to surpass in height Salisbury Craigs and Arthur's Seat" 

 (p. 52) ; while the other stream might be traced from Lon- 

 don to the coasts of Devon and Dorset. The description 

 given (pp. 104-106) of the volcanic outburst in the island 

 of Sumbawa can hardly be read without our feeling that 

 we know of no one ancient volcanic rock comparable in 

 extent to the deposit of ash which must have resulted 

 from the single eruption of 1815. 



After having shown that these tremendous effects of 

 volcanic action on the surface are " insignificant when 



