18 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. IMbmo,b 'voTxix. 



and longitudinal fissures, accompanied by energetic eruptions of porphyrite, partly as dikes, 

 partly as widespread subaerial flows of pyroxene-andesite, and labradorite-porphyry and 

 diabase, of generally gabbroidal composition. 



Orogenic and eruptive activity recommenced in about the middle of Carboniferous times. 

 The region was now dry land, and there was built up a mass of igneous rocks, tuffs at first largely 

 acidic with keratophyric and aplitic types, followed by a mass of pyroxene-and labradorite- 

 porphyrites, and diabases, and small intrusive masses of diorite, syenite, and granite. The 

 folding and metamorphism of the western portion of the Urals occurred about this time, and was 

 apparently accompanied by the immense plutonic intrusions which have the form of lenticular 

 and probably essentially laccolitic masses, consisting of peridotite, pyroxenites, gabbros, diorites, 

 and granites. There are two chief types of complexes; in the one the olivinic rocks predominate, 

 and about it are a series of shells of rocks, the acidity increasing towards the periphery, which 

 consists of gneissic hornblende-gabbros showing secondary dynamometamorphism. The ultra- 

 basic rocks of the interior are free from dykes of the gabbro or diorite. (Fig. 5). In the 

 second type of occurrences, pyroxenites predominate over olivinites, which occur merely as 

 narrow schlieren-bands, the gabbros are proportionately more abundant and the whole series 

 is less regularly arranged. In both types of occurrence the diorites may pass into granites and 

 finally gneissic albite-granites, the marginal portions being always strongly banded or schistose. 

 The intrusion of acid rock appears generally to have followed that of the basic rocks, among which, 

 however, the sequence is less constant, and some repetition of ultrabasic intrusions seem to have 

 occurred. In general, however, the sequence is one of increasing acidity from below upward, 

 from within outwards, and usually from the west eastwards. The separation between the various 

 types of rocks is sometimes sharp; at other times there are all gradations between them. 



According to Bowen's view this complex suggests a sheet-like mass dipping eastwards 

 with the peridotite and pyroxenite near its base, and passing eastward (and upward), suc- 

 cessively less basic rocks. The pyroxemte-bodies occurring as borders about the dunites 

 (collected olivine-crystals) are considered to be huge reaction-rims. The size of these bodies, 

 however, would seem to demand very efficient diffusion in the magma, were they formed in this 

 manner. 



This complex illustrates the general nature of the occurrences of ultrabasic rocks in the 

 southern Urals. According to Duparc ('14, '16) they form an enormous band stretching for 

 hundreds of kilometers along the Eurasiatic divide. The individual occurrences consist usually 

 of elliptical masses of homogeneous dunite, surrounded by a continuous or interrupted band of 

 olivine-bearing pyroxenite passing into koswite. This varies greatly in amount, and is in turn 

 surrounded by a zone of gabbro, which is the most abundant of the three rocks. Generally the 

 mass of dunite occurs eccentrically in the pyroxenite and gabbro. One instance has been 

 described in which it is in direct contact with the gabbro-diorite and schists, and only a small 

 amount of pyroxenite-tilaite and olivine-gabbro occurs in association with this mass. Contrary 

 to Wyssotsky's statement, Duparc declares that in all cases observed in detail the separation 

 of the three main types of rock is sharp, without any lateral passage. 



In the regions of the Ural Mountains further to the north somewhat similar conditions occur 

 as has been shown by Duparc and Tikanowitch ('14) in the region of the Wagran and Kakwa 

 rivers. Here the sedimentary sequence is much less complete, being chiefly composed of infra- 

 Devonian quartzites and schists. There are intrusive into these large masses of gabbro, in 

 comparison with which the pyroxenites play a very secondary role. They also occur in distinct 

 masses surrounding the dunite-serpentines intrusive into the green schists. Near Lake Turgojak, 

 the serpentines and amphibolites of the Urals are invaded by alkaline syenite-porphyries (Vis- 

 con t-' 13) and large masses of alkali-syenites, etc., occur elsewhere in the range. 



GERMANY, BOHEMIA, AND NORTHERN HUNGARY. 



The present state of uncertainty as to the age of the crystalline masses of Saxony and south 

 Germany prevents a satisfactory consecutive treatment of the earlier igneous action and tectonic 

 history of this land. The definitely Palaeozoic complex consists of Cambrian, Ordovician, and 



