June i r, 1908] 



NA TURE 



159 



iiigf its growing acceptance, though many will still hesitate 

 before his conclusions (p. 373) as to the profound modifica- 

 tion of a gabbro-magma by absorption of pre-existing acid 

 masses. He admits, with other writers, a " gravitative 

 differentiation of the compound magma of assimilation," 

 and urges that at each stage in the intrusion of the 

 Okanagan batholith a magma more basic than the average 

 of the rock invaded became enriched with silica by 

 assimilation, and also bv this gravitational draining away 

 of the denser material. Dynamic and accompanying hydro- 

 thermal actions are said to have produced gneisses from 

 the intrusive Okanagan " granodiorite," and it is held 

 that in many places mineral material, especially from the 

 more basic constituents, " has been leached out from the 

 granulated rock and has re-crystallised in strong shear 

 zones to which the solutions have slowly travelled " (com- 

 pare pp. 345-6). This would seem in accord with 

 Lehmann's view of the development of biotite along shear 

 zones ; and, as even a tentative e.xplanation of the forma- 

 tion of a strikingly banded gneiss out of a homogeneous 

 "granodiorite," it has considerable petrographic interest. 



Messrs. E. C. Andrews and J. C. H. Mingayc, assisted 

 by the careful petrographic descriptions of Mr. G. W. 

 Card, discuss the granites of northern New England in 

 part iv. of their description of the New England plateau 

 of New South Wales (Records Geol. Survey N.S.W., vol. 

 viii., 1907, p. 196). They conclude that their granitic 

 masses are mostly " bathyiiths." At Hillgrove they find 

 a diorite (p. 232) that passes insensibly through a fine- 

 grained granite rich in biotite into silicified slates, which 

 further pass into true black slates. " The biotite-granite 

 is thus possibly a compromise between the slates and the 

 diorite." The authors urge (pp. 234 and 237) that the 

 " bathyiiths " have occupied their present positions at the 

 expense of the older rocks, and they are advocates of 

 sloping as a means of carrying away derived material. 

 In part v. of the memoir [ibid., p. 239), Mr. .Andrews 

 explains the structure of the deposits of wolfram, 

 cassiterite, and molybdenite as a " replacement of granite 

 by [the products of] solutions and gases rising along the 

 intersecting joints " (p. 241), and spreading at times from 

 "an incredibly small core." Mr. Card contributes 

 " Mineralogical and Petrographical Notes, No. 10 " [ibid., 

 p. 2J7), referring his rocks to the American system of 

 classification, on the basis of the thorough analyses pro- 

 vided by Mr. Mingaye's laboratory. We hope that some 

 day these notes may be brought together into a general 

 petrographic study of the colony, so that we mav learn 

 the part that each rock has played in the broad sequence 

 of geological events. 



A fine example of that rare rock, orbicular granite, is 

 described from the Transvaal Bushveld by Mr. W. J. Gau 

 (Trans. Geol. Soc. of S. .Africa, vol. ix., 1906, p. 70). 



The mining district of Pitkaranta on Lake Ladoga, in 

 Finland, has been thoroughly investigated by Mr. Otto 

 Triistedt from a mineral and petrographic point of view 

 (Bull, de la Comm. geol. de Finlande, No. 19, 1907). 

 His fine memoir, written in German, of more than 300 

 pages, lays special stress on three zones of " skarn " — a 

 somewhat forbidding word in Swedish — which here means 

 a calciphyre formed from crystalline and usually dolomitic 

 limestones. These limestones are part of an ancient 

 amphibolitic and sedimentary series, now highly meta- 

 morphosed, into which granites and pegmatites intruded in 

 early pre-Cambrian times, followed in much later, but 

 still pre-Cambrian, times by the great mass of Rappakiwi- 

 granite. The " skarn," chiefly composed of salite and 

 garnet, is believed (p. 91) to have arisen through the 

 influence of magmatic waters circulating from the earlier 

 intrusive bodies. Earth-movements then broke it up in 

 many places into blocks, round which singular modifica- 

 tions have taken place. Serpentinisation, acting through 

 a long period, has spread inward from their surfaces (e.g. 

 pp. 218 and 235), producing an eozoonal structure, built 

 up of salite, serpentine, and calcite. This banded struc- 

 ture is well shown in the photographic plates and figures. 

 1 he Rapp.-ikiwi-granite ultimately invaded the wdiole series 

 of rocks, and became solid. During the last phases of 

 the eruptive activities of which it forms the climax, metallic 

 ores, including cassiterite, w-ere deposited in its zone 

 of contact. Magnetite, zinc-blende, and sulphides of iron 



NO. 2015, VOL. 78] 



and copper new began to replace certain layers of the 

 "eozoon," and the outer portions of some of the lumps 

 of "skarn," or " Salitaugen," pass into pure ore. Even. 

 garnets (p. 138) have been dissolved away along certain 

 of their zones of growth, and these zones have been re- 

 placed by metallic minerals. The author agrees with Vogt 

 (P- 3>5)'that the Pitkaranta ore-deposits belong to the true 

 contact-metamorphic type, and occupy a middle position 

 between the iron and copper deposits of Christiania and 

 the tin and copper deposits of Cornwall. 



Mr. Curt Fircks's paper, written in English, on the 

 occurrence of gold in Finnish Lapland (jbid.. No. 17, 

 1906), describes ferruginous veins traversing granulite as 

 the mother-lodes of the alluvial gold ; but it is not clear 

 why they are called (p. 33) " a new type of gold veins, 

 not yet observed in other parts of the world." 



Messrs. Johnson and Warren (.American Journal of 

 Science, vol. xxv., 1908, p. i) revive our interest in Wads- 

 worth's " cumberlandite " in their account of the geology 

 of Rhode Island. This rock becomes a " rhodose " in 

 accordance with the new system of classification (p. 25) ; 

 it has the high specific gravity of 4, owing to its ground- 

 mass of inter-grown magnetite and ilmenite, enclosing 

 hyalosiderite (p. 19), an olivine rich in iron. The olivine 

 has retained grains of the iron ore within it, and Mr. 

 Warren suggests that these were prevented from joining 

 the main mass by the decreasing mobility of the separating 

 olivine, and he favours the supposition that the minerals 

 became immiscible before their freezing- point was reached 

 (P- 22). 



An unusual group of rocks, for which ultra-basic 

 intrusive masses seem primarily responsible, is described 

 from the A'izagapatam district of Madras by Messrs. T. L. 

 Walker and W. H. Collins (Records Geol. Survey of 

 India, vol. xxxvi., 1907, p. i). The ultra^jjasic igneous 

 border of a mass of granulites and garnetiferous granites 

 (charnockite series) is believed to have mingled with 

 sillimanite-schists. Rocks rich in green spinel and' 

 magnetite thus pass into others where the spinel is largely 

 replaced by sapphirine, and sapphirine is regarded (p. 11) 

 as a product of the interaction of spinel and sillimanite. 

 .A mineral hitherto described as hypersthene in the 

 charnockite series, with a pleochroism " sky-blue to red 

 or red-brown " (p. 14), is shown to have frequently oblique 

 extinction. The authors seem to throw doubt on the exist- 

 ence of " a rhombic pyroxene with the properties usually 

 assigned to hypersthene"; but surely they mean merely to 

 ask whether rhombic pyroxene ever has a "sky-blue" 

 axis-colour. 



Before leaving ultra-basic igneous rocks, it may be 

 mentioned that Dr. Corstorphine further defends his view, 

 criticised in South .Africa and elsewhere, as to the con- 

 cretionary nature of the eclogite masses in the diamond- 

 pipes of'Kimberley (Proc. Geol. Soc. of S. Africa, 1907, 

 p. Ixi). The tenth volume of the Transactions of the Geo- 

 logical Society of South Africa, which these Proceedings 

 accompanv, contains contributions by Messrs. Voit, 

 Merenskv] and J. P. Johnson on the same subject, and 

 an interesting rejoinder by Prof. T. G. Bonney to Messrs. 

 Corstorphine and' Voit, all being included in the part for 

 Julv to December, 1907. 



Mr. L. L. Fermor, in describing rhyolites and basalts 

 from Pdv.4gad Hill, Bombay Presidency (ibid., vol. xxxiv., 

 1906, p. 148), points out differences between the former 

 and the Malani rhyolites of Rdiputana described by Mr. 

 La Touche, in spite' of some points of striking resemblance. 

 In consequence, he is able to suggest that the P.4vAga(I 

 rocks, both basic and acid, were poured out as differentia- 

 tion-products from one caldron in Cretaceous times, the 

 alternative being that the basalts form intrusive sills in a 

 far older rhvolitic series. .A vertical section of 2400 feet of 

 rock is exposed, to which Mr. Fermor invites the attention 

 of future visitors who can undertake its detailed explora- 

 tion. 



From five weeks' studv in the field, Mr. James Currie 

 has drawn up an illustrated list, arranged topographically, 

 of the minerals in the basaltic F.-croes, which will 

 especially appeal to students of zeolites (Trans. Edinburgh 

 Geol. -Soc, vol. ix., 1006, p. i). 



'Coming now to sedimentary deposits. Herr Meigen has 

 continued his researches on calcium carbonate, w^hich have- 



