February. 1911. 



KNOWLEDGE. 



71 



cases no account is taken of the relative masses ot the rocks 

 represented by the analyses, in others these relative masses are 

 reckoned with as far as possible. By the former method the 

 average igneous rock appears to have a composition close to 

 that of many pyroxene andesites and quartz-mica-diorites : in 

 passing it is interesting to note that these are extremeh' conunon 

 rock types. By the second method Professor Vogt finds the 

 average rock to be far more acid — with about 74% of silica — 

 and thus corresponding with a granite in this respect. In " An 

 Introduction to Petrology" (19101, Mr. F. P. Mennell tackles 

 the subject by this second method, dealing with the rocks mapped 

 in the neighbourhood of Bulawayo, in Rhodesia. Putting 

 minor rock types into one or other of larger groups, he estimates 

 eleven thousand nine hundred and sixty square miles of igneous 

 rock (assumed to extend one mile vertically downwards) to 

 be made up as follows : — Granite, eleven thousand six hundred 

 and seventy cubic miles : syenite, forty ; picrite, two hundred 

 and forty ; dolerite and basalt, ten ; and the average silica 

 percentage becomes nearly seventy. If this is fairly repre- 

 sentative of other large areas of the earth, it would indicate 

 that granite represents the average composition of the igneous 

 rocks. 



A somewhat similar problem is attacked by R. .A.. Daly in 

 " The Average Chemical Composition of Igneous Rock Types" 

 (Proc. Anier. Acad. Arts and Sciences, xlv., 1910'. The 

 author, using various collections of analyses, calculates the 

 average chemical composition of ninety-eight principal igneous 

 rock types, and incidentally these may be used as a basis for 

 finally calculating the " average igneous rock." The relative 

 uniformity in the soda percentage of the more abundant types 

 is specially noted in its bearing on the origin of oceanic 

 sodium, and therewith, on the problem of the age of the earth. 

 The striking similarit\- of the axerage granite analysis to the 

 average analysis of the base (ground mass) in augite andesite. 

 and the equally close resemblance of the average diorite 

 analysis to the arithmetical mean of a\erage basalt and granite, 

 are illustrated. 



The same author discusses the " Origin of the Alkaline 

 Rocks" in Bulletin Geol. Soc. America, xxi. (1910). It is 

 usually recognised that there are two great branches of igneous 

 rocks — the " alkaline " (rich in soda and potash), corresponding 

 in a general way with the areas of the Atlantic type of coast- 

 line .as defined by Suess, and the '" sub-alkaline " or " lime- 

 alkaline," corresponding with the Pacific type. In the present 

 paper. Dr. Daly states that no alkaline province can be 

 described .as free from sub-alkaline eruptives. especially those 

 of basaltic or granitic types. Emphasis is laid on the 

 indisputable fact that the visible volume of all alkaline rock- 

 bodies is a very minute quantity as compared with the visible 

 volume of sub-alkaline eruptive bodies. An inductive study 

 shows that most alkaline rocks cut thick masses of limestone 

 or other calcareous sediments. This fact suggests the 

 hypothesis that the absorption of carbonate disturbs the 

 chemical equilibrium of sub-alkaline magma in such manner 

 that alkaline fractions are produced by differentiation. Most 

 of the alkaline species are ascribed to the interaction of 

 basaltic magma and limestone (or dolomite), but more acid 

 magma is also sensitive to the solution of carbonate. The 

 hypothesis explains the concentration of alkalies, the desilica- 

 tion shown by the crystallisation of nepheline, leucite, 

 corundum, and so on ; the extreme variability of alkaline 

 bodies in mineralogical and chemical composition, the 

 occurrence of such lime-bearing materials as melilite, scapolite. 

 melanite. and so on, and COj-bearing minerals, as cancrinite 

 and primary calcite. 



,\lkaline rocks of exception.al interest are found in Ayrshire, 

 and neighbouring parts of Scotland. .\ preliminary account 

 of some of the types is supplied by Mr. G. W. Tyrrell {Trans. 

 Geol. Soc. Gtasgoic. vol. xiii.. pt. iii.), who promises a more 

 detailed paper on the subject. Teschenite. essexite and 

 trachyte are among the types, and some of the rocks contain 

 much analcime, which appears to be primary. Thus there is 

 an analcime syenite, composed principally of soda-orthoclase. 

 albite and analcime, with purple titaniferous augite. barkevi- 

 kite and aegerine. Another rock is composed principally of 

 analcime, with a little nepheline, crow ded with perfect euhedral 



barkevikite, sometimes with a little titaniferous augite and 

 plagioclase. 



In the Geological Magazine for Januar\% 1911, Mr. F. P. 

 Mennell describes dolerites of Rhodesia, containing quartz, 

 either in separate granules or in the form of niicropegmatite. 

 The dolerite dykes penetrate the great .granite masses of the 

 country, and the quartz, when occurring in good-sized corroded 

 fragments, is obviously derived from the granite. In the case 

 of the smaller granules and the micropegmatite, the origin of 

 the quartz is not so clear. The phemonena are, in some 

 respects, closely similar to those which Professor Judd recorded 

 in certain cases, which seemed to him to prove the secondary 

 origin of micropegmatite and the growth of crystals in rock's 

 after their solidification (O.J.G.S.. May, 1889). In the 

 Rhodesian rocks there is clear evidence that the micro- 

 pegmatite fringes represent the sur\ iving portions of crystals 

 which have had their exteriors melted or corroded. " The 

 outgrowths are, therefore, as Judd correctly surmised, of 

 the nature of secondary enlargnients long after the consoli- 

 dation of the original nucleus. Where his suggestion was at 

 fault, was in regarding the regrowth as having taken place at 

 the expense of a non-crystalline ground mass. The outgrowths 

 that we have been discussing are clearly due to the re-crvstal- 

 lisation, under the influence of heat, of the exterior portions of 

 the original crystals, together, in some cases, w ith materials due 

 to reactions with the surrounding crystals or with introduced 

 substances. The light such an observation throws on the 

 occurrences of quartz and micropegmatite among basic rocks, 

 and upon the frequently-noted association of gabbro or 

 dolerite with granophyre. is evident. There seems little doubt 

 that it may usually be ascribed to the partial admixture, prior 

 to intrusion, of acid and basic materials, not necessarily from 

 related magmas, or even entirely of igneous origin." 



Very similar rocks to these Rhodesian quartz and micro- 

 pegmatite-bearing dolerites were described in the same 

 magazine in July and -August, 1909, in Mr. G. \V. Tyrrell's 

 " Intrusions of the Kilsyth-Croy District." These rocks, from 

 Dumbartonshire. Scotland, are described as granophvric 

 diabase, and the grains of quartz and patches of micro- 

 pegmatite are said to be primary. It is here suggested that 

 the gabbro-granophyre melange rocks owe their origin to the 

 interaction of a normal basalt-magma with a highly siliceous 

 country rock, and that the normal granophyric diabases, 

 with their remarkably constant chemical composition, represent 

 the saturation-point of such a magma with silica. The excess 

 of siliceous matter is believed to be thrown out as a separate 

 body of material, usually consolidating as granophyre, in a 

 manner analagous to the separation of the excess of a salt in 

 a saturated solution. 



While dealing with rock-magmas and solutions, reference 

 may be made to a paper in the American Journal of Science 

 for January, 1911, — "Solid Solution in Minerals, with Special 

 Reference to Xephelite," by H. W. Foote and W. M. Bradley. 

 It is a well-known fact that there are certain minerals to 

 which no satisfactory chemical formulae can be assigned, 

 which agree with the results of analysis. In general, the 

 composition of a mineral, as obtained in analysis, \aries from 

 the composition of the ideal pure compound for two reasons, 

 apart from errors of analysis. Either there is la I isomorphous 

 replacement of one element or radical by another, or ibj 

 there are mechanical impurities present. The authors call 

 attention to another influence, which must probably be 

 taken into account in cases like that of nepheline (to which 

 the formulae XaAlSiOj and Na<.-\lsSi<iOM. besides others 

 more complicated, have been given). It is assumed that 

 in certain cases a substance on crystallising forms a solid 

 homogeneous solution with foreign matter, which is neither 

 isomorphous with any constituent, nor can be regarded as a 

 mechanical mixture. It can be compared to the solution of 

 salt in water, in which the salt takes on the appearance and 

 form of the water without taking any part in the formula of 

 the water. The authors maintain that the pure compound, 

 which forms the basis of nepheline, is the orthosilicate 

 Xa.AISiOj. and that the observed variations from this are 

 due to silica dissolved in the silicate. Just as very little is 

 known about the condition of dissolved substances in liquids, 



