70 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. fMEMOm *YoTxix: 



area, which are not associated with clearly plutonic rock types, or those elsewhere with pillow- 

 structure or possibly of tuffaceous origin, as products of the crystallization of a magma thrust 

 into consolidated Jurassic sediments during folding, and under a thick cover of Upper Jurassic, 

 Cretaceous, and possibly Eocene deposits. Under such circumstances plutonic and hypabyssal 

 features only would occur. It seems, therefore, preferable to accept the views of those who 

 consider such masses as approximately contemporaneous with the associated sediments being 

 intrusions into unconsolidated material or even submarine flows, and to note, moreover, the 

 geosynclinal conditions of their development and their often spilitic chemical character. Where, 

 however, such rocks are intimately associated with plutonic types, other considerations may 

 be noted. Plutonic intrusions of peridotite and gabbro are not infrequently followed by dikes 

 of diabase, and in a strongly dislocated region the three types might become intermingled and 

 their relations obscured. We may conceive it to be possible that the basic magma rising along 

 a plane of shearing in a geosynclinal zone may be pressed out at the surface in front of the 

 advancing overthrust anticline or crust-flake, and may consolidate in the form of submarine 

 volcanic rock, massive or tuffaceous, and as the movement continues, these may become over- 

 ridden by the advancing sheet; the magma passing along the thrust-plane may now be injected 

 into the previously formed volcanic rocks, and thus a plutonic series of intrusions, with possibly 

 some differentiation may invade volcanic rocks, perhaps rendered more or less schistose by the 

 later movements. The plutonic rocks may later be invaded by a few dikes. In this way, 

 perhaps, we may explain the features of the Italian and Californian green-rocks, with pillow- 

 lavas, peridotite and gabbro invaded by diabase or even locally by veins of granite, and on 

 the other hand, the agglomerates and breccias with serpentines, etc., of Cretaceous age, which 

 surround the Indian massif, though it must be recognized that in the case of those to the west 

 of India, the descriptions and diagrams given by Vredenburg ('09) do not lend much support 

 to such a suggestion. The difficulties in the problem of the origin of such associations of igneous 

 rocks are so great, that the above-suggested explanation, which embodies some features of 

 Steinmann's hypothesis, is put forward with much diffidence. Kober's ('21) remarks on the 

 eruption of the green-rocks in the alpine orogenic regions during intermittent folding and over- 

 thrusting in Mesozoic and early Tertiary times seems to be largely in accord with this sugges- 

 tion. His terse phrase (p. 279) which contrasts these rocks from those considered below may 

 be translated thus: "The orogenic vulcanism in the time of great mountain-building, leads 

 to the formation of metamorphosed volcanic rocks, among which the green-rocks play an im- 

 portant r61e. All the rocks of this orogenic phase appear to be of Pacific types. In sharp 

 contrast to these stand the rocks of the geosynclinal phase, the time of quiet sedimentation 

 in the geosyncline. There the Atlantic magma appears." While this seems too sweeping in 

 its assertion of sharp contrast, the fact that it is often difficult to draw the line between the 

 two tectonico-petrographical groups, does not appear to the writer to render any less real the 

 tendency of the rocks developed to vary systematically with the tectonic conditions in the 

 direction indicated. From the very nature of the case, rocks of intermediate characters 



could be expected. 



THE SPILITIC ROCK-SUITE. 



The reality of this suite as a definite tectonico-petrographical group has been challenged by 

 Daly ('14, p. 38S). He points out the difficulty in drawing a clear distinction between the 

 normal basalts and diabases in Germany, and those which show alkaline characters, which we 

 have already indicated, and we might also refer to the apparent similarity in history of the 

 strongly albitized rocks in the southwest of England and the much less frequently albitized 

 rocks of Germany, though the latter are also included in the spilitic suite by Flett and Dewey 

 ('11). In place of the explanation put forward by these authors, which is accepted by Harker 

 ('17), who holds the spilitic suite to be a special division of the alkaline branch of igneous rocks, 

 Daly adopts and extends Bowen's ('10) suggestion concerning the origin of albitic rocks. The 

 albitization is not the result of the post-volcanic replacement of originally basic feldspar by 



