386 



KNOWLEDGE. 



October, 1913. 



more or less incidental are considered and the size and import- 

 ance of the various ice-streams taken into account, the 

 impression is produced that a longer period of study is 

 necessary, and in most cases more detailed records are 

 required. Those who make general statements covering the 

 phenomena of the whole globe raise not unfounded suspicions 

 that they are looking too keenly for unreasonable uniformity. 



In Switzerland, if we eliminate small cirque-glaciers, which 

 are usually in a state of oscillation, only the Lower Grindelwald 

 glacier and the small glacier of the Sex Rouge show continued 

 advance (Annales de Geographie, July, and so on). In 

 Scandinavia in several important instances advances up to 

 one hundred and sixty metres are recorded ; but, on the whole, 

 the glaciers are in a state of oscillation, and no general statement 

 is justified. In Asia, New Zealand, and South America definite 

 information is wanting, but elsewhere it is stated there is a 

 general tendency to regression. In Alaska, however, certain 

 of the main glaciers have pushed forward their ice-fronts, 

 that of La Perouse, for example, having gained four hundred 

 metres. This growth followed upon earth tremors recorded 

 in 1899, and has been correlated with them. Hess prefers to 

 connect the advance with the abundant snowfalls of the end 

 of the nineteenth century. 



Joseph Vallot (Comptes Rendus, CLVI, May, 1913,) gives 

 a record of temperature observations in glaciers at high 

 altitudes (four thousand metres) made during 1898, 1900, 

 1911. A diurnal variation up to 13° (centigrade) is observable 

 to a depth of -7 metre. Down to 7-5 metres the temperature 

 falls rapidly, and below that it varies between —12° and —13°, 

 remaining stationary at —12 -8° below fifteen metres. At the 

 depth of about one metre, it may be concluded, diurnal 

 temperature variation ceases, and the limit of the seasonal 

 variation is 7 • 5 metres. The grain of the ice grows from 

 ■ 5 millimetre to 1-2 millimetres in diameter, apparently 

 without fusion, and the ice is impermeable. Hence neither 

 infiltration of water nor temperature variation, which ceases 

 at a relatively insignificant depth, can be an important factor 

 in producing the flow of glaciers. 



By G, 



GEOLOGY. 



VV. Tyrrell, A.R.C.Sc, F.G.S. 



GARNET AS A GEOLOGICAL BAROMETER.— The 

 petrological investigation of a peculiar manganiferous igneous 

 rock (kodurite) discovered by Dr. L. L. Fermor, of the 

 Geological Survey of India, in connection with his recent 

 survey of the manganiferous deposits of the Indian Empire, 

 has led to certain conclusions as to the mode of formation of 

 garnet (Records, Geological Survey of India, Vol. XLIII, 

 Part I, 1913). Typical kodurite is a plutonic rock consisting 

 of orthoclase, spandite (manganese garnet), and apatite. 

 Associated with kodurite is a garnet (spandite) rock. In 

 attempting to classify these rocks Dr. Fermor calculated their 

 analyses into the norm or standard mineral composition of 

 the American Quantitative Classification. The norm of 

 kodurite contained orthoclase, leucite, anorthite, hedenbergite, 

 wollastonite, tephroite, magnetite, ilmenite, and apatite ; whilst 

 that of the garnet rock was expressed as a mixture of anorthite, 

 hedenbergite, akermanite, fayalite, tephroite, and magnetite. 

 A comparison of the specific gravity of the mode (actual 

 mineral composition) with that of the norm (standard mineral 

 composition) showed that the kodurite occupied ten per cent., 

 and the spandite rock twenty per cent., less room when 

 crystallised as the mode than as the norm. This indicated 

 at once that the crystallisation of kodurite and spandite rock 

 was conditioned by high pressure, necessitating a decrease in 

 volume as compared with the conditions of pressure under 

 which the norms might have crystallised from the respective 

 magmas. 



If this be the true interpretation, garnet may be regarded 

 as a geological barometer, indicating especially high pressure 

 during its formation. Confirmation is found in the fact that 

 garnet is an abundant constituent of all the various rocks 

 associated with the kodurite series. Further speculation on 

 these lines suggested that eclogite is the high-pressure form of 



gabbro, and consideration of the chemical analyses and the 

 specific gravities of these rocks supported this view. Since 

 the plutonic rocks, formed at great depths under pressure, are 

 typically non-garnetiferous, Dr. Fermor postulates the existence 

 below the plutonic zone of a shell characterised by garnets 

 wherever the necessary sesquioxide radicle is present. For 

 this shell is suggested the term infra-plutonic. Other 

 minerals, notably diamond, the high-pressure form of carbon, 

 may also be characteristic of the infra-plutonic zone, and 

 Dr. Fermor makes the interesting suggestion that the diamond 

 pipes of Kimberley, in which eclogites and garnets are found, 

 may be directly connected with the infra-plutonic zone. 



PRESERVATION OF PLANT-FOSSILS IN LAVA.— 

 Some interesting examples of tree- moulds in lava are described 

 by F. A. Perrett (American Journal of Science, Vol. 

 XXXVI, August, 1913) from Kilauea. The basalt lava from 

 this great crater has sometimes invaded a forest of trees and 

 then flowed away, leaving an investment or casing of lava 

 upon a tree-trunk up to a height corresponding with the 

 greatest depth of the lava stream at that point. These are 

 known as the salient or projecting types of tree-mould, and 

 stand above the surrounding plain as a monument to the 

 original tree, which, if not destroyed at once by the basalt- 

 flood, soon dies and rots away. 



In the sunken or ground type of tree-mould the lava has 

 invaded low ground, and has come to a standstill among the 

 trees. The latter, of which no trace now remains, have left 

 an impression of their forms down to the minutest detail upon 

 the lasting stone. These casts are found on the sides of 

 cylindrical openings, from three to five metres deep, in the 

 lava-plains. An excellent illustration in the paper shows the 

 faithfulness of these impressions of the rugged bark of the 

 trees upon the plastic lava. It is a matter for comment that 

 the trees were not destroyed by their " baptism of fire," long 

 before such impressions could be taken. If, however, the 

 resinous varieties be excepted, large tree-trunks, massive and 

 full of moisture, will resist carbonisation at least long enough 

 for a skin of cold, solid basalt to form in contact with them, and 

 thus provide a nearly non-conducting layer. 



Similar plant-remains preserved in lava have been described 

 by Solorzano and Hobson from Mexico, where fragments of 

 maize were found in basaltic scoriae ; and by H. M. Cadell, 

 who has described a lycopod stem, twelve inches long, 

 embedded in olivine-basalt lava from the Bo'ness Coalfield. 

 Phenomena identical with those described by Perrett were 

 observed by F. A. Fouque on Etna, where the lava of the 

 1865 eruption flowed through a wood of lofty trees. 



SAPPHIRE IN MULL.— Clear blue corundum has been 

 found by the Geological Survey in two localities to the west of 

 Carsaig, Mull. The mineral occurs in tabular hexagonal 

 plates, which are, however, too small and irregular to have 

 any gem value. In each case the crystals are found in 

 igneous rocks which have involved xenolithic masses of sedi- 

 mentary material. In one locality large xenolithic blocks of 

 baked sandstone and shale are involved in an igneous matrix 

 which is probably intrusive, and which encloses numerous 

 little sapphires. In the second locality the sapphires are 

 found in an irregular composite sill consisting partly of 

 andesitic felsite and partly of trachytic granophyre or syenite. 

 This intrusion is also full of xenolithic material, including 

 baked shale. The sapphires are found both in the latter and 

 in the igneous matrix. 



METEOROLOGY. 



By William Marriott, F.R.Met.Soc. 



BEN NEVIS OBSERVATORY.— It is announced that a 

 tourist hotel is being built on the top of Ben Nevis on the site 

 of the former Meteorological Observatory. This hotel will 

 probably be the terminus of the four and a half miles railway 

 which it is proposed to construct when funds are assured and 

 sanction obtained to proceed. 



It will, no doubt, be remembered that the Ben Nevis 



