16 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON, lMEM0IE ^ TI xix: 



their clastic (sometimes protoelastic) structure and sill-like habit. It is not clear, however, 

 whether this was a pre- Cambrian or early Caledonian orogeny, though the latter appears more 

 probable. 



Interesting observations have been made also in northern Norway by Foslie ('21). "Out- 

 side the real roots of the mountain-chain, all eruptives seem to have been intruded parallel 

 to the schists, the moving force being induced by the orogeny folding itself, and lateral pressure 

 existed throughout the crystallizing period." Where the marginal meshwork of crystals was 

 able to protect the central residue of uncrystallized magma, a complex was produced by dif- 

 ferentiation in situ, consisting of marginal norites including scattered masses of peridotite, 

 and a sharply distinct central body of quartz-norite, with aplite-dikes. There is here no effective 

 gravity-control of the differentiation. Where the lateral pressure was excessive, there are 

 sill-like masses of rather sodic amphibolite with albitized plagioclase, and small lenticular 

 masses of altered peridotite, together with sodic granite or granodiorite, and rarely schlieric 

 types of diorite. 



While there is thus in Norway an association of alkali-calcic rocks with alkaline granites, 

 Goldschmidt ('16) holds that one "would be going too far in generalizing from such always rare 

 associations if one denied the essential difference between most alkaline stems and alkali-calcic 

 stems. Direct comparison between the alkali-calcic stems of the Caledonian folded mountains 

 and the completely alkaline stem of the neighbouring fault-graben of Christiania affords an 

 instance of this." Brogger ('94) showed that the latter rocks, some of which are basic, were 

 erupted during the Devonian block-faulting, and they may therefore be classed in our alkaline 



plateau-group. 



RUSSIA. 



The great chain of the Ural Mountains contains almost throughout its length a discon- 

 tinuous series of broadly lenticular masses of basic rocks (as shown by the International 

 Geological Map 1892), but the writer has not been able to obtain much information concerning 

 its geological history and tectonics. Suess (Vol. Ill, p. 400) considers that it resulted from the 

 posthumous folding of the Eurasian nucleus, and that the main movement occurred in Carbon- 

 iferous times, and was a thrusting toward the west. Haug groups the range as a geosynclinal 

 area ('08); Duparc and Pearce ('03) remark on the evidence of several periods of folding; but 

 the present ranges result from a re-elevation in Tertiary times of an ancient folded region (Cole 

 '14). It was in connection with the Carboniferous folding, however, that the development of 

 the great intrusive masses occurred. The sedimentary succession as displayed on the eastern 

 flank of the range near Ekaterinburg shows Middle Devonian limestone and radiolarian rocks 

 associated therewith. With these are basic rocks possibly of a spilitic nature, " andesites," 

 vesicular porphyrites, diabase-porphyrites, and tuff with breccia (Lagorio and Karpinsky '97, 

 Tschernychew '87). Upper Devonian limestone and slates are followed conformably by 

 Carboniferous conglomerates. Recently Wyssotsky ('13) has given an exceptionally interest- 

 ing description of the whole structure of the northwest and southwest of Ekaterinburg, which, 

 combined with the above information, gives a comprehensive account of the later palaeozoic 

 history of the Ural Mountains in the region lying between 57° 30' and 59° 4' north latitude. 

 Here the Urals are zonal mountains, asymmetrically folded, and dipping isoclinally westwards. 

 The geological history of this region, so far as we are concerned, commences with the eruption 

 of basic magma into the clayey sediment at the commencement of the Devonian period. On 

 the western flanks of the range these have become albiti'c green-schists and schalsteins; while 

 those erupted in Lower and Middle Devonian times on the eastern slopes appear as diabase 

 and porphyrite, tholeitic and vitrophyric andesites, with acid plagioclase and a large amount 

 of keratophyre. In other words, the rocks of the spilitic suite are developed in a fairly typical 

 manner. In other parts of the Ural Mountains these volcanic eruptions continued into early 

 Carboniferous times. Their relations are obscured by later folding which has sometimes 

 converted the keratophyre into a sericitic schist. They were at first submarine flows, which 

 directly followed the depositions of the Lower Devonian limestone, vesicular lavas occurring 



