January ly, 1901 1 



NATURE 



277 



their speculations are of a crude and tentative nature. 

 It is greatly to be desired that students of physical 

 chemistry should turn their attention to petrology ; the 

 more so since some subjects, such as the nature of 

 solutions and the constitution of alloys, which have 

 recently been advanced in their hands, may be found to 

 have important applications to the crystallisation of 

 igneous rock-magmas. 



Petrological research during the last few years serves 

 especially to emphasise the value of comparative studies 

 of different areas. There are large regions throughout 

 which the igneous rocks show but relatively slight de- 

 partures from one common type, while elsewhere com- 

 paratively small districts exhibit a surprising range of 

 variation. Peculiar rock-types, supposed to be unique 

 and to have the most narrowly restricted occurrence, 

 have in some instances been found to recur at widely 

 separated centres with the same associates and with like 

 geological relations. More generally there are differences 

 as well as resemblances, the rocks of two districts con- 

 stituting two parallel suites, such that each type in the 

 one suite has its representative in the other. Such a 

 parallelism is that drawn by Brogger between the Mon-- 

 zoni and the Christiania rocks. It points clearly to two 

 somewhat similar, but not identical, parent-magmas hav- 

 ing undergone differentiation on similar lines. The in- 

 vestigations of the United States Geological Survey, and 

 of other American geologists, afford numerous illustrations 

 of these and other points. The intrusive masses in the 

 High Plateaux region along the west side of the Rocky 

 Mountains, furnishing, as Gilbert first showed, such 

 beautiful examples of the laccolitic form, are, according 

 to Cross, almost wholly of diorite, diorite-porphyrite, &c. 

 The region lying along, and to the east of, the Rocky 

 Mountains, from Montana to Texas, differing widely from 

 the former belt in geological structure, is equally in con- 

 trast with it petrographically. The rocks here are 

 generally richer in alkalies, and they embrace a remark- 

 able profusion of types and varieties. Especially is this 

 seen in the accounts given by Weed and Pirsson of the 

 . several mountain-groups of Montana. Some of the 

 rock-types described are new and unique, such as mis- 

 sourite, a leucite-gabbro without felspar ; but most of 

 them compare with rocks already known from other 

 areas, though usually presentmg points of difference 

 which may be significant. The memoir by Cross and 

 Penrose on the Cripple Creek district, Colorado, may 

 also be cited in the same connection. This district and 

 some of the others alluded to are important mining 

 centres, and it is very interesting to notice how economic 

 geology and what may be termed pure petrology assist 

 one another. Some of the economic questions raised, 

 such as the source and origin of the Montana sapphires 

 (Pirsson, Amer. Joicrn. Sci. 1897, vol. iv. p. 421), maybe 

 found to have no unimportant application to the chemistry 

 of igneous rock-magmas. 



Since it is not possible in a short article to give even a 

 summary of the actual results of petrological work during 

 late years, we confine ourselves to a few examples. In 

 Britain one result of recent researches, by the Geological 

 Survey and other workers, has been to reveal the occur- 

 rence of a number of rock-types hitherto but little known, 

 or wholly unrecorded, in this country. Among them are 

 rocks of the syenite and nepheline-syenite families and 

 the related families of dyke-rocks, some closely compar- 

 able, and probably contemporaneous, with the remarkable 

 suite of Devonian intrusions of the Christiania basin. 

 Especially interesting is Teall's brief account of the in- 

 trusive masses of Cnoc na Sroine and its vicinity, in the 

 western part of Sutherland. The main mass is found to 

 consist of a quartz-syenite of the nordmarkite type, which 

 graduates on the one hand into a granite, on the other 

 into quartzless syenite, nepheline-syenite, and the rock 

 formerly described under the name borolanite. The 



NO. 1629, VOL. 63] 



last-named rock is composed essentially of orthoclase 

 and melanite garnet with some aegirine-augite and 

 alteration-products of nepheline. A peculiar feature is- 

 the occurrence in it of polygonal pseudomorphs doubtless 

 representing leucite, and the rock is practically identical 

 with the so-called leucite-syenite of Magnet Cove in 

 Arkansas. The associated minor intrusions — sills and 

 dykes — are partly of dark hornblendic rocks approach- 

 ing camptonite, partly of light felspathic rocks containing 

 aegirine, and comparable in different varieties with the 

 grorudite, lindoite, &c., of the Christiania district. The 

 distribution and petrography of the Scottish lampro- 

 phyres and the peculiar felspathic rocks which seen> 

 to be their natural complements are as yet im- 

 perfectly known ; but Flett, Trans. Roy. Soc. , Edin. 

 (iQoo) vol. xxxix., p. 865, has described from ' the 

 Orkneys dykes of bostonite, camptonite, monchiquite and 

 alnoite, and some of these types are known to be repre- 

 sented in various parts of the Highlands. Camptonites. 

 and augite-camptonites are described by Hill and Kynas- 

 ton in Argyllshire, where they are genetically related to 

 another remarkable rock-type, kentallenite, which in its 

 association of alkali-felspar with olivine and augite re- 

 sembles the olivine-monzonite of Predazzo. The kental- 

 lenite itself is related to, and occurs in part as a marginal 

 facies of, the large granite and tonalite masses of the 

 district. This is only one example of the very hetero- 

 geneous nature of the intrusions marked on the geological 

 maps of Scotland as granite. The same variability 

 characterises them from the Galloway "granites," recently 

 described by Teall in a Survey Memoir, to the large 

 intrusion of Aberdeen, where the extreme basic modifica- 

 tions are represented by troctolite and peridotite (bastite- 

 serpentine). An interesting group of minor intrusions, as- 

 yet only partially described, includes the orthophyres, 

 lamprophyres, and quartz-basalts of South Devon. Finally 

 we may mention a unique rock described by Judd 

 {Trans. Roy. Irish Acad. (1897) vol. xxxi. p. 48), and 

 named rockallite after the the remote islet where it occurs. 

 It is a peculiar granite-porphyry consisting of oegirine, 

 quartz and albite ; differing from Brogger's grorudite, and 

 from other acid rocks, in its remarkable richness in iron 

 and poverty in alumina and in the absence of potash. 



One point worthy of remark is the way in which various 

 crystalline rocks, to which more or less of obscurity has at- 

 tached, are being reclaimed from the limbo of " gneisses," 

 " granulites," &c., and recognised, some as true products 

 from igneous fusion, others as metamorphosed sediments. 

 A recently published memoir by Holland {Mem. GeoL 

 Sun'. India (1900), vol. xxviii.) establishes the igneous- 

 origin, and describes the petrographical characters, of an 

 important group of Archaean rocks in southern India. 

 We may note incidentally, as illustrating the increasing 

 hold which the idea of genetic grouping is obtaining among 

 petrologists, that the author boldly uses the name " char- 

 nockite series " for an assemblage of types ranging in 

 composition from acid to ultrabasic. Their community- 

 of origin is attested not only by their intimate association 

 but by remarkable points of resemblance which run 

 through the whole series. The constant presence of 

 hypersthene in the rocks is one characteristic, and the 

 common acid type, to which the name chamockite is 

 given, is in fact a hypersthene-granite. The component 

 minerals exhibit an astonishing freshness of preservation. 

 Since rocks generally similar have been described, under 

 such names as pyroxene-gneiss, pyroxene-granulite, &c., 

 as occupying very extensive areas in Peninsular India, 

 Ceylon, and Burma, a proper appreciation of their nature- 

 and origin is a matter of considerable importance. 



Microscopical research and chemical analysis, as- 

 representing, on the side of the laboratory, the ground- 

 work of all our knowledge of rocks, must necessarily- 



j retain the important position which they occupy ; and in. 



I both fields improved methods are coming into general- 



