104 



KNOWLEDGE 



[June 1, 1891. 



state in which many of its remains are preserved, to have 

 accumulated with considerable rapidity. Fishes are found 

 with their scak's undisturbed, cidarids with their spines 

 in situ and so forth, showing that they were entombed in 

 material solid enough to support them before the destruc- 

 tion of their soft parts was accomplished. There is also 

 reason to think, from the absolutely amorphous state of 

 much of the calcareous matter, that some of it was of 

 chemical or mechanical origin ; that is, it is either a 

 invcipitate from solution or else detritus from pre-existing 

 limestones. Dr. Sorby considers it impossible for calcite 

 shells to form such a substance by their destruction, though 

 aragonite ones may do so. 



Such are the principal arguments which have been 

 adduced m denial of the theory that the Chalk was an 

 earlier equivalent of the Atlantic Ooze. If, as geologists 

 are becoming more inclined to hold every day, that theory 

 is untenable, what shall we put in its place ? 



According to Dr. A. R. Wallace, conditions almost 

 identical with those under which the Chalk was formed 

 exist at the present time in the northern part of the Gulf 

 of Mexico. The researches of Pourtales show that the 

 ocean bed is there covered with a fine white mud, closely 

 resembling Chalk in composition, and which consists chiefly 

 of the impalpable detritus of the coral-reefs which fringe 

 the numerous islands, together with the skeletons of the 

 foraminifera abounding in the warm waters of the region. 

 Coral-reefs exist in the Chalk of Maastricht and Faroe, 

 but with these exceptions they are almost unknown in the 

 formation. 



Prof. Prestwich also discusses the subject at considerable 

 length in his " Geology." He thinks that much further 

 investigation will be required in order to set the question 

 at rest. In the meantime, he is of opinion that the Chalk 

 was formed under conditions which have passed away, or 

 at any rate are nowhere exactly realised at the present 

 time. The stratigraphy indicates that it was deposited in 

 a nearly enclosed sea of no great depth. The rivers 

 flowing into this sea brought down a very exceptional 

 amount of soluble silica, though not so much as in some- 

 what earlier times : some of the Upper Green-sand beds 

 contain as much as 72 per cent. In the presence of solid 

 silica (of which sponge spicules, Ac, are made) or of organic 

 matter, this substance was precipitated, and formed the 

 flints which so often enclose remains of sponges, or occupy 

 the tests of echinoderms. 



WHAT IS THE CAUSE OF VOLCANIC ACTION? 



By Kev. H. N. Hutchinson, B.A., F.G.S. 



IN our previous paper we endeavoured to explain the 

 structure of a Volcano, and described briefly the 

 chief phenomena of an eruption. " Is it possible," 

 the reader may ask, " to form any conclusions as to 

 how volcanic eruptions are brought about ? " — in 

 other words, to find out what is going on underneath, and 

 so to obtain some idea of the cause or causes of these 

 strange manifestations of subterranean activity. It must 

 be confessed at once that, in the present state of scientific 

 knowledge, no full and complete explanation is possible. 

 Geologists and others are as yet but feeling their way 

 cautiously towards the light which, perhaps before long, 

 will illumine the dark recesses of this mysterious subject ; 

 but nevertheless, since volcanic action was first carefully 

 studied by Mr. Scrope, Sir Charles Lyell, and others, such 

 valuable material has been collected, that we are getting 

 much nearer to a true theory now that the ground has 



been somewhat cleared. Others, among living geologists, 

 have carried on researches of very great value, and so have 

 thrown valuable light on the subject. It will, perhaps, 

 hardly be necessary to point out that the main difficulty is 

 our ignorance of the interior of the earth. If wo could 

 penetrate subterranean regions to a sufficient depth, and 

 find out the physical conditions prevailing far below the 

 surface, there would be little difficulty in finding out how 

 Volcanos are worked. But since direct knowledge is 

 impossible, the problem must be attacked indirectly. We 

 are somewhat in the position of a medical man diagnosing 

 a difficult case ; only medical science has the great 

 advantage of knowing accurately the internal structure of 

 the human body. The earth, unfortimately. is a body the 

 internal anatomy of which is unknown. Of its epidermis, 

 or skin, we have learned a good deal, but beyond that all 

 is speculation. Looking upon volcanic action as a curious 

 disease from which our patient the world occasionally 

 suffers, it may not be unprofitable to see if some rough 

 sort of diagnosis of the case is possible. 



For this purpose it will be necessary to consider volcanic 

 action a little more generally. We must not confine our 

 attention to any one outbreak of the disease, or to any one 

 Volcano, but look at the subject as a whole, putting our- 

 selves, as it were, in the position of the physician who 

 judges of any one "case" from the experience he has 

 derived from the study of a great many " cases." 



Now the first thing to remark is that volcanic action 

 goes through phases, of which there are three. First, 

 there is the state of permanent eruption — this is not a 

 dangerous state, because the steam keeps escaping all the 

 time the safety-valve is working, and all goes on smoothly. 

 The second state is one of moderate activity, with more 

 or less violent eruptions at brief intervals — this is rather 

 dangerous ; the safety-valve is at times jammed. And 

 thirdly, we have paroxysms of intense energy alternating 

 with long periods of repose, sometimes lasting for centuries. 

 These eruptions are extremely violent and cause widespread 

 destruction ; the safety-valve has got jammed and so the 

 boiler bursts. No Volcano has been so carefully watched 

 for a long time as Vesu\'ius. Its history illustrates the 

 phases we have just mentioned. The first recorded erup- 

 tion is that of A.D. 7S, a very severe one of the paroxysmal 

 type, by which the towns of Hereulaueum, Pompeii, and 

 8tabi» were buried up. We have an interesting account 

 by the younger Pliny, whose uncle lost his life through 

 remaining too near the scene of action, partly for the sake 

 of rescuing those in danger and partly because he wished 

 to observe the strange phenomenon. Before this great 

 eruption took place Vesuvius had been in a quiescent state 

 for 800 years, and if we may judge from Greek and Iloman 

 writings was not even suspected of latent possibilities in 

 the way of volcanic eruptions. Then followed an interval 

 of rest till the reign of Severus, the second eruption taking 

 place in the year 203. In the year 472, says Procopinus, 

 all Europe was covered, more or less, with volcanic ashes. 

 Other eruptions followed at intervals, but there was com- 

 plete repose for two centuries, i.e. until the year 1306. In 

 1500 it was again active, then quiet again for 130 years. 

 In 1631 there came another paroxysmal outburst. After 

 this many eruptions followed, and they have been frequent 

 ever since. Vesuvius is, therefore, now in the second stage 

 of moderate activity. 



But geologists can take a wider view of Volcanos than 

 this ; their researches into the volcanic action of remote 

 geological periods have yielded important results, which 

 may be briefly indicated here. They can sum up the 

 history of a volcanic region, and the result seems to be 

 somewhat as follows : — There is a regular cycle of changes ; 



