442 



KNOWLEDGE. 



Xovemb?:r, 1911. 



ijiven of tlie present ideas upon the subject, and it is sliown 

 how certain conflicting notions may be reconciled. 



Other important contributions are devoted to the 

 ■' Absorption Spectra of Metallic Vapours." by Dr. Be\an. 

 ""The Compressibility of Mercury," by Dr. McLewis; and 

 ""The Use of Indicators for determining .Affinity Values," by 

 Dr. \'. H. \'eley. There is also an interesting paper on "The 

 Theory of Colloids," by Professor Freundlcr. in which lie 

 classifies colloidal (or glue-like) solutions into two groups. \i/.. 

 those in which the solid body, e.g.. a colloidal metal, is in a 

 state of extremely fine suspension, termed " Suspension 

 Colloids " or "" Lyophobic Sols " ; and " Kmulsion Colloids " 

 or ■■ Lyophilic Sols " (including gelatin solutions and the like) 

 which approximate more closely to true solutions. 



In a paper upon "' The Treatment of Wheaten Flour." Mr. 

 .A. Humphries describes the results of his investigation of the 

 changes produced in flour on baking. From these it appears 

 that a large proportion of the organic phosphorus present is 

 converted into an inorganic form. 



ALUMINIUM NITRIDE AND ITS USES. — On 

 heating aluminium powder in the air it absorbs oxygen and 

 nitrogen in proportions depending upon the temperature 

 reached. Thus, at 600' C. oxygen is absorbed up to 8-S per 

 cent, of the weight of aluminium, but there is little or no 

 absorption of nitrogen ; while at SOO'C. the amount of nitrogen 

 absorbed is trifling. On raising the temperature to llOO'C, 

 rapid absorption of both oxygen and nitrogen takes place, until, 

 after about six hours, the metallic aluminium has been con- 

 verted into oxides and nitride, the absorbed nitrogen being 

 subset|uently displaced by oxygen, with the final production of 

 a hard aluminium oxide resembling alumina in properties but 

 containing a different proportion of oxygen. 



When the aluminium is heated in an atmosphere of 

 nitrogen at 900°C., it absorbs 12-21 of that element, to form 

 a nitride, which is decomposed into an alnminimum oxide 

 when heated in the air at temperatures abo\ e ,sn()°C. 



In the current issue of the Bull. .Assoc. Chiiii. Siicr. ct Dtst. 

 11911, X.XVIII, 10101, M. Kohn-Abrest recapitulates his work 

 on this subject, outlined above, and suggests that it may be 

 possible to prepare aluminium nitride on a large scale from 

 bauxite, or alumina, in the electric furnace. The products 

 would contain from ii to 34 per cent, of nitrogen, and in the 

 case of bauxite would cost less to make than sodium nitrate, 

 containing' 15-5 per cent, of nitrogen. Further experiments 

 are needed, however, to ascertain whether ammonia could be 

 produced economically on a large scale from this source : or 

 whether, if applied to the soil as a fertilising agent, aluminiimi 

 nitride, as thus prepared, would part with its nitrogen in the 

 form of ammonia. 



GEOLOGY. 



By G. W. Tyrrell, A.K.C.Sc, F.G.S. 



(JU.ART/ \'i:iNS. — Whilst in manv cases undoubtedly 

 due to deposition from water containing silica in solution, it is 

 now thought that some quarts veins may ha\'e an igneous 

 origin. The " aqueous " veins generally have a " comb " or 

 other regular structure growing out from the sides of the 

 fissure in which they have been deposited. Quartz veins of 

 igneous origin have a coarse pegmatoid structure, and may 

 show graduations in mineralogical composition uniting them 

 with typical pegmatites. The latter are admittedly igneous, 

 and represent the final product of the slow crystallization of 

 the more volatile residual portions of a granitic or syenitic 

 magma. These mother-liquors, rich in magmatic water, were 

 forced into fissures of the already-solid magma, or into the 

 adjacent country-rock, where their extreme liquidity permitted 

 the formation of very large and perfect crystals of the later- 

 crystallized constituents of the magma, such as quartz and 

 orthoclase. (Juartz veins associated with granite masses may 

 be regarded as pegmatites eompo.sed entirely of quartz, and 

 there is no reason why they should not have the same origin 

 as the more normal pegmatites. 



It is to be remarked that the distinction between '" aqueous " 

 and " igneous " becomes very shadowy in the case of quartz 

 veins. In discussing pegmatite Mr. Harker remarks: "The 

 magma or solution, from which the pegmatites crystallised 

 was igneous, in that it was the residual part of a granitic or 

 syenitic or other igneous magma, of which the greater part 

 had already crystallised under plutonic conditions. It was 

 .aqueous, inasmuch as it contained, perhaps very richly, 

 magmatic water, concentrated (with other volatile constituents) 

 in the residual magma by continued crystallization of 

 anhydrous minerals. The pegmatites themselves represent 

 this watery residual magma, except that the greater part of 

 the water and other volatile substances was expelled in the 

 final crystallization .... Logicallv, indeed, we might 

 include under the same head simple quartz veins crystallised 

 from solution in water (at perhaps 200'-'C), if both quartz and 

 water were of direct intratelluric origin, the final residuum of 

 an igneous rock-magma." 



Some observations bearing on the origin of quartz veins 

 in association with a granite mass have "oeen made by 

 ^\^ C. Simmons, in a paper on the Foxdale Granite of 

 the Isle of Man [Gcol. Mag., August, 1911). In both the 

 larger granite mass at Foxdale, and the smaller one of Eairy, 

 thick \eins of quartz and co.arse pegmatite occur. Sometimes 

 these quartz veins, then resembling dykes, are found, outside 

 the granite, intruded into the country rock. .Associated with 

 them and with the granite are characteristic dykes of micro- 

 granite intruded into the slates with a N.W. — S.E. trend. 

 The veins occur in greatest number on the south-side of the 

 intrusion, and attain a maxinunn thickness of eighteen feet. 

 In the large Foxdale Silica Ouarry several veins are to be 

 seen, which pass locally into a quartz-mica pegmatite. In 

 one case a vein has a selvage, half-an-inch thick, consisting of 

 a felted mass of bronzy mica ; in another the selvages, again 

 half-an-inch thick, are composed of a mixture of quartz and 

 orthoclase. The evidence seems to support the theory that 

 the quartz veins represent the final consolidation product of 

 the granite magma. The seh'ages are believed to be due to 

 the segregation of impurities from the molten silica solution. 

 There is no sign of " comb " or any other structure to 

 indicate that the seins are due to vapour or water deposition. 



PETROI.KArHlCAL I'R'JVINCES.— In hi^ I'residential 

 .Address to the Geological Section of the British .Association 

 at Portsmouth, Mr. Harker took as his subject "" Some Aspects 

 of Modern Petrology." Most of the address was concerned 

 with what one may call " regional petrology" — the space and 

 time distribution of igneous rocks. This is a fascinating sub- 

 ject, and its study is opening up some wide problems, and 

 gi\ing not a few hints as to that natural classification of 

 igneous rocks which belongs to the future, but which at present 

 is merely hoped for. Petrographical provinces are, in Mr. 

 Harker's words, " more or less clearly defined tracts, within 

 which the igneous rocks, belonging to a given period of 

 igneous activity, present a certain community of petro- 

 graphical characters, traceable through all their diversity, or 

 at least obscured only in some of the more extreme members 

 of the assemblage." It is natural to attribute such a family 

 likeness amongst a given group of igneous rocks to a 

 community of origin. The simplest hypothesis is that which 

 holds all the varied types of a petrographical province to 

 have been evolved by the progressive differentiation of an 

 originally homogeneous parent-magma. Becker holds the 

 view that petrographical provinces are the expression of a 

 primaeval heterogeneity of the earth's crust, which has 

 persisted throughout geological time. This is diflicult to 

 reconcile with the small extent and sharp definition of 

 provinces like that of .Assynt or the Bohemian Mittelgebirge. 

 An even stronger objection is that petrographical provinces 

 are not permanent. In the Midland \'alley of Scotland, for 

 instance, a calcic petrographical province (Old Red Sand- 

 stone) is followed by one distinctly alkaline (Carboniferous), 

 extending over almost exactly the same area. Daly's theory 

 of the origin of alkaline rocks as due to the assimilation of 

 limestone by calcic magmas, is open to the same serious 

 objections. 



