A HALF-CENTURY OF SCIENCE. 81 



time, in many cases, to a wide and deep circular opening at the top of 

 the cone, called by the advocates of this hypothesis a "crater of ele- 

 vation." 



This theory, though, as it seems to us now, it had already received 

 its death-blow from the admirable memoirs of Scrope, was yet that 

 most generally adopted fifty years ago, because it was considered that 

 compact and crystalline lavas could not have consolidated on a slope 

 exceeding 1 or 2. In 1858, however, Sir diaries Lyell conclusively 

 showed that in fact such lavas could consolidate at a considerable 

 angle, even in some cases at more than 30, and it is now generally 

 admitted that, though the beds of lava, etc., may have sustained a 

 slight angular elevation since their deposition, still, in the main, vol- 

 canic cones have acquired their form by the accumulation of lava and 

 ashes ejected from one or more craters. 



The problems presented by glaciers are of very great interest. In 

 1843 Agassiz and Forbes proved that the center of a glacier, like that 

 of a river, moves more rapidly than its sides. But how and why do 

 glaciers move at all? Rendu, afterward Bishop of Annecy, in 1841 

 endeavored to explain the facts by supposing that glacier-ice enjoys a 

 kind of ductility. The " viscous theory " of glaciers was also adopted 

 and most ably advocated by Forbes, who compared the condition of 

 a glacier to that of the contents of a tar-barrel poured into a sloping 

 channel. We have all, however, seen long, narrow fissures, a mere 

 fraction of an inch in width, stretching far across glaciers a condi- 

 tion incompatible with the ordinaiy idea of viscosity. The phenome- 

 non of regelation was afterward applied to the explanation of glacier- 

 motion. An observation of Faraday's supplied the clew. He noticed 

 in 1850 that, when two pieces of thawing ice are placed together, they 

 unite by freezing at the place of contact. Following up this sugges- 

 tion, Tyndall found that, if he compressed a block of ice in a mold, it 

 could be made to assume any shape he pleased. A straight prism, for 

 instance, placed in a groove and submitted to hydraulic pressure, was 

 bent into a transparent semicircle of ice. These experiments seem to 

 have proved that a glacial valley is a mold through which the ice is 

 forced, and to which it will accommodate itself, while, as Tyndall and 

 Huxley also pointed out, the " veined structure of ice " is produced by 

 pressure, in the same manner as the cleavage of slate-rocks. 



It was in the year 1842 that Darwin published his great work on 

 " Coral Islands." The fringing reefs of coral presented no special dif- 

 ficulty. They could be obviously accounted for by an elevation of the 

 land, so that the coral, which had originally grown under water, had 

 been raised above the sea-level. The circular or oval shape of so many 

 reefs, however, each having a lagoon in the center, closely surrounded 

 by a deep ocean, and rising but a few feet above the sea-level, had 

 long been a puzzle to the physical geographer. The favorite theory 

 was, that these were the summits of submarine volcanoes on which the 

 VOL. xx. 6 



