434 



KNOWLEDGE. 



NOVKMBER, 1912. 



AssiiiiiinK th.it tin- Ir-Huiii prcsiiit in these K'lses w,is 

 oriKin.illy dcrivfd from the disiiitcKnition of radioactive sub- 

 stances, it appears prob.abic that it has not been produced 

 immediately before its emission, but that it consists of gas 

 which may have been formed some time previously from 

 miner.ils in the vicinity and then dissolved by the water. 



It is noted as a curious fact that these and other springs 

 emitting gases rich in helium follow the course of a line 

 extending through Moulins, Dijon and Vesoul. 



GEOLOGY. 



Hy G. W. Tyrrell, A.R.C.Sc, F.G.S. 



EARTHQUAKES AND GLACIERS.— A hitherto unsus- 

 pected relation between earthquakes and glaciers has been 

 disclosed by recent work in the Alaskan region. This is fully 

 set forth in Professional Paper No. 69 of the United States 

 Geological Survey. During September, 1899, the region of 

 Yakutat Hay, Alaska, was shaken by a scries of severe earth- 

 quakes, which were attended by two notable results — great 

 changes in the level of the land incidental to faulting, and 

 remarkable subsequent changes in the adjacent glaciers. Both 

 these remarkable effects entitle this earthquake to be regarded 

 as a type, and as one of the great earth(juakes of the world. 



The changes of level are the greatest recorded in historical 

 times. The maximum uplift, attested by raised beaches and 

 other phenomena, amounts to over forty-seven feet. Similar 

 measurements of vertical displacements are numerous along 

 the fjords of the region ; and not only faulting, but extensive 

 tilting and warping of a complicated character have been 

 demonstrated by these means. Barnacles attached to ledges 

 high above present tide-marks, and amongst land shrubs ; 

 mussel shells still .ittached to rocks a score of feet above sea- 

 level, afford clear evidence of very recent uplift. 



Most of the Alaskan glaciers that have been studied shewed, 

 at least up to 1905, distinct evidences of recent recession. 

 Those of Yakutat Bay were found by Professor R. S. Tarr 

 and L. Martin, the authors of the report, to be all in a state 

 of recession when examined in the summer of 1905. In 1906 

 Professor Tarr returned to the region, and found a most 

 astonishing change. In the intervening ten months many of 

 the glaciers had advanced hundreds of feet, their smooth, 

 moraine-covered surfaces were broken into a jagged sea of 

 seracs and crevasses, and all were notably thickened. 



This sudden, spasmodic advance is attributed to the effects 

 of the earthcjuake of 1899. The Yakutat Bay glaciers are 

 nourished by the normal accumulation of snow and ice over 

 a vast region overhung by craggy mountain slopes on which 

 great masses of snow are lodged in precarious situations. 

 There is first-hand evidence that the great shaking of 

 September, 1899, threw down enormous (|uantities of snow 

 and ice into the gathering-grounds, and caused what must be 

 regarded as an ice-flood, analogous to a river flood. The 

 latter arrives in the lower reaches of the river in a few hours 

 or days; the ice-flood causes a great advance at the foot of 

 the glacier in a few years, the exact time being determined by 

 the rate of flow and the length of the ice. Confirmation of 

 this simple and beautiful explanation is found in the fact that 

 the first glaciers to advance were the shortest. The longest 

 of the Yakutat Bay glaciers have not yet responded to the 

 earthquake shaking. Furthermore " the advance was alike in 

 several respects in all the glaciers — it was abrupt and 

 spasmodic, it caused profound transformation of the glacier 

 surface, and it resulted in thickening at the termini — and all 

 the glaciers quickly subsided and returned in a few months to 

 a stagnant state after the effects of the rapid forward move- 

 ment were spent." 



The puzzling fluctuations of the termini of glaciers are 

 usually ascribed to climatic variations; and this is probably 

 the correct explanation in many cases. In earthquakes, 

 however, we have provided an alternative cause of oscillation, 

 undoubtedly operative in Alaska, and probably also in other 

 ylacicr regions of the world. 



PETROLOGY AT THE BRITISH ASSOCIATION.— 

 Petrologists were in force at the Dundee meeting, and many 

 interesting papers were read and discussed. 



The seventh and final report of the committee on the 

 composition and origin of the crystalline rocks of Anglesey 

 summarizes a valuable piece of work. In all, forty-three 

 complete rock analyses and twenty- five partial analyses have 

 been made, which alone constitutes a notable addition to 

 British petrography. In this report, a final series of analyses 

 is given, mostly of metamorphic rocks ; but one of a " variolitic 

 pillowy diabase " has also been made, which is of interest in 

 comparison with other spilttic lavas, and in view of the 

 interest now taken in the " spilitic suite." 



A most interesting discussion followed Dr. Flett's paper on 

 " The Sequence of Volcanic Rocks in Scotland in relation to 

 the Atlantic and Pacific Classification of Suess." The paper 

 dealt mainly with four great groups of volcanic rocks: 11) The 

 Carboniferous, typically " Atlantic " or alkaline in type, with 

 olivinebasalts, trachytes, phonolitcs, mugearites, and later on 

 teschenites, monchicjuites and nepheline basalts ; (21 The Old 

 Red Sandstone, consisting mostly of andesites, and with 

 typical " Pacific " or calcic characters ; (3) The great Tertiary 

 suite, unquestionably of .'Vtlantic affinities, notwithstanding the 

 Pacific types clustered around certain local centres ; (4) The 

 " spilitic suite," consisting of pillow-lavas and their associates, 

 occurring in the Dalradian schists (Tayvallichi, and in the 

 Ordovician of the Southern Uplands (see " Knowledge," 

 February, 1912. page 75, and April. 1912. page 152). The 

 spilitic suite is considered by Dr. Flett to constitute a great 

 division of igneous rocks equivalent in value to the .Atlantic 

 and Pacific divisions. 



The discussion turned mainly on the validity of the .Atlantic 

 and Pacific classification, and on the status of the spilitic 

 suite. The nomenclature especially of the former w;is 

 attacked. Great confusion had been caused, it was alleged, 

 by the use of the terms Atlantic and Pacific in a petrographic 

 sense. They should be restored to their rightful owners, the 

 tectonic geologists and geographers, and petrologists should 

 revert to the terms '" alkalic " and " calcic " for their two main 

 divisions. Mr. A. Harker was a notable convert to Dr. Flett's 

 view of the spilitic suite. 



Mr. G. Barrow described a case of " magmatic differentiation 

 intensified by dynamic action" in the older granite of Deeside. 

 The granite permeates the crystalline gneisses over large 

 areas in lit-par-lit fashion, forming minute sills, which vary 

 from an inch to several feet in thickness. These have pro- 

 ceeded from dyke-like masses exposed in the hillsides. As the 

 sills are traced towards their taper end. oligoclase and biotite 

 steadily diminish in amount, their place being taken by 

 alkaline felspar and muscovite. There is a concomitant 

 increase in grain-size, the taper ends of the sills thus taking on 

 a pegmatitic character. It is clear that under great pressure, 

 the more acid, residual, liquid magma has been separated from, 

 and driven farther into the gneisses, than the less acid, already 

 crystallized material with which it was originally entangled. 



Dr. \V. Mackie described vesicular rhyolites with beautiful 

 flow-banding, occurring round the Ord Hill of Rhynie, 

 .Aberdeenshire. These flows rest on an eroded surface of the 

 diorites and gabbros of West -Aberdeenshire — rocks which are 

 considered to represent basic modifications of the younger 

 (jrampian granites. The rhyolites are faulted .against Old 

 Red Sandstone and are thought to be older than the oldest 

 beds of this series. It seems to the present writer, however, 

 that these rhyolites have a distinctive Old Red Sandstone 

 facies, and may prob.ably be correlated with similar rocks in 

 the Lome area. 



The present writer described the alkaline igneous rocks of 

 .Ayrshire, which were recently dealt with in this column 

 (" KNOWLEncE." September 1912, page 342*. .An additional 

 point of petrographic interest is the demonstration that the 

 zeolitic mineral thomsonite may be of primary igneous origin, 

 since it is found in the syenite of Howford Bridge, Mauchline, 

 with exactly the same ch.aracters that give analcite so strong a 

 claim to be considered as primary in this rock. 



