186 



KNOWLEDGE. 



[August 1, 1899. 



of horses. Some walking by the side of the cattle are 

 oonstantly stretching out their beaks to the horses' head 

 or jumping up to its belly, in order to snap off a fly or 

 tick which they have spied ; others perched on the backs of 

 the beasts relieve them of many an enemy. All the while 

 the cattle never flinch, but seem to know perfectly well 



Fig. 4. — Avocet's Eggs in a Scoop in the Mud. 



that the birds with their graceful movement and dehcate 

 peck are doing them a service. 



Squacco herons,* with reddish-brown backs, were also 

 in these marshes, as well as many flocks of that most 

 beautiful of all the heron tribe — the little egret, f The 

 entire plumage of this lovely bird is of a pure snowy white, 

 its beak and legs are black, and its eyes yellow. Like all 

 the herons it has long plumes on the head and breast, 

 and growing from the middle of its back, and drooping 

 over its wings, are those filamentous wavy feathers so 

 exquisitely beautiful on the bird — so artificial and unsightly 

 on a woman's bonnet. 



SECRETS OF THE 



By Grenville A. J. Cole, 



EARTH'S CRUST. 



M.S.I.A., F.G.S., Professor 

 Oeology in the Royal GolUge of Science for Ireland, 



OF 



of 



IV.^k CALDEON OF THE ROCKS. 



THE phenomena of volcanoes, so full of mystery, so 

 fraught with perilous excitement to those who 

 dwell upon their flanks, have been disguised in a 

 haze of erroneous statements, and in much poetic 

 imagery, born of astonishment and fear. To this 

 day, such expressions as " the smouldering of subterranean 



• Ardea ralloidet. 



■f Ardea garzetta. 



fires," or " vast clouds of incandescent gas," are by no 

 means confined to the daily press, or the pages of sensa- 

 tional magazines. Sir AVilliam Hamilton,'' the well-known 

 Envoy of His Majesty at the Court of Naples, may be 

 said to have set the example, so far as English observers 

 are concerned, in systematic observation, day by day, of 

 what actually goes on in a volcano. His views on the 

 structure of volcanic cones anticipated those of Scrope,t 

 and his researches, in a large degree, preceded those of 

 his Italian contemporary, Spallanzani. In later years, 

 our ideas have been kept, as it were, from wandering, by 

 the successive editions of Sorope's memorable works, and 

 by the series of papers from the pen of his successor, 

 Prof. J. W. Judd.J Prof. Judd, in his book on 

 " Volcanoes, "§ has summarised a long series of observa- 

 tions ; while Sir A. Geikie|j has recently called attention 

 to the debt we owe to the great French geologists in the 

 early years of the present century. The latter author has 

 also added, in his history of volcanic action in our islands, 

 to the large debt we owe to his own writings. 



If the elucidation of the visible phenomena of volcanoes 

 still remains a subject for discussion, far greater diflSculties 

 are sure to be experienced when we try to realise what is 

 going on beneath the surface. What happens in the great 

 underlying lava-basin, in this unseen caldron of the rocks ? 

 What evidence have we as to its nature ? How can we 

 hope to make any actual observations ? 



In the venerable, but highly erroneous diagrams that 

 may still be found in atlases of physical geography, a 

 section of the earth's crust may be seen, in which the 

 whole stratified series is piled up, like a sloping row of 

 books, and resting at one end upon a primitive mass of 

 granite. The high antiquity of granite, and its eminently 

 fundamental character, are conceptions which date back 

 to the last century, and which were not then to be lightly 

 called in question. 



Yet, when James Hutton,*! in 1785, made his famous 

 discovery in Glen Tilt, and showed that granite sent off 

 veins into the surrounding rocks, its supposed fundamental 

 character was successfully assailed. The theory of the 

 deposition of all crystalline rocks from the waters of a 

 primitive ocean was at one time widely accepted, and was 

 held by some Italian geologists down to very recent 

 years. Yet, from Hutton onwards, observation after 

 observation showed that such rocks were intrusive in 

 others at their margins, and were at times, therefore, of 

 later date than Mesozoic or Cainozoic rocks.** If, as was 

 thus generally proved, granites and their kindred were at 

 one time in a state of fusion, with what type of igneous 

 phenomena were their features most nearly allied ? 



The modern volcano does not erupt granite, or diorite, 

 or gabbro, or any material in a highly crystalline condi- 

 tion. The fused and viscid lava, when it consolidates, 

 may even form a glass, imperfect, it is true, but certainly 

 glassy to the eye. The surface of highly siliceous lava- 

 flows, like the Rocche Rosse of Lipari, resembles the 

 broken refuse of a bottle-factory ; while the smoother and 



* " Observations on Mount Vesuvius, Mount Etna, and other 

 Volcanoes," 1772; Second Edition, 1773 ; also the superbly illustrated 

 folio on the Campi Phlegrtei, 1799. 



+ "Considerations on Volcanoes," 1825. 



X "Contributions to the Study of Volcanoes," Oeol. Mag., 1876. 



§ International Scientific Series, 1881. 



II " Ancient Volcanoes of Great Britain " (1897), Vol. I., preface. 



% See Lyell, "Principles of Geology," Vol. I. (1830), p. 62; 

 Hutton, " Theory of the Earth," Vol. III., now first published in 1899. 



•* See, for instance. Knowledge, Vol. XXI., p. 124. 



