72 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. IMemo,r ^o"xix; 



in the majority of cases show clearly that they were erupted during block-faulting of the crust, 

 unaccompanied by strong lateral compression. The absence of such compression, perhaps 

 even the tension in the one case and the steady downward sagging of the crust in the other, 

 may be conditions so far analogous as to accord with the grouping of the spilitic suite as a 

 special class of the alkaline rocks, as has been done by Harker. We may therefore look upon 

 the picrites (paleopicrites) which sometimes form sills clearly comagmatic with rocks of the 

 spilitic suite, as comprising a definite tectonico-petrographic group of ultrabasic intrusions. 



THE ALKALINE PLATEAU-GROUP. 



The picrites that accompany the essexitic-theralitic eruptive rocks are not distinguishable 

 petrographically from those of the spilitic suite, and we have already indicated above the 

 reason for considering that these two groups are closely allied in tectonic as well as petrographic 

 features, but that, nevertheless, they are sufficiently different to form distinct subdivisions 

 of the alkaline branch which are not, however, necessarily sharply separated. In the case of 

 the rocks of this group, therefore, the conditions under which crystallization occurred have 

 been an absence of effective lateral pressure associated with an unusual abundance of volatile 

 constituents in the magma; so that here circumstances have been especially favorable for the 

 development of gravitational differentiation, and the production of such complexes as the 

 Shonkin Sag laccolite or the recently described Lugar sill in Ayrshire (Tyrrell '16). From 

 such magma-chambers also, picritic lavas may be poured out. It is clear, however, that ultra- 

 basic rocks developed in such an association have a very different significance from those which 

 occur in the normal peridotite-gabbro-granite complexes. (Cf. Daly, '14, p. 451.) 



THE ALKALmE-PERTDOTITE DIKES. 



In considering the peculiar dikes of mica-peridotite in India, South Africa, and in the United 

 States, we have recognized yet another group of ultrabasic intrusions of definite tectonico- 

 petrographical characters, having been developed under much the same regional tectonic con- 

 dition as the last-mentioned series, though forming not sills but dikes and rarely small stocks. 

 In Arkansas these mica peridotites are probably coeval with the alkaline complex of Magnet 

 Cove, and occur in the same tectonic province. We may consider them to be specialized mem- 

 bers of the alkaline plateau group. Adding to the series we have mentioned above, the dikes 

 of melilite-basalt in Tasmania, and the diamond-bearing "pipe" at Minas Gereas in Brazil, Du 

 Toit ('20) points out that these all occur in the foreland-regions of great folded arcs, and sug- 

 gests that they may be derived from the ultrabasic differentiate of dolerite-magma still remain- 

 ing in the subcristal reservoir, into which it had been thrust out from the folding-zone in the 



manner described below. 



THE DOLERITE-SILLS. 



Du Toit ('20) shows that the dolerite-sills of the Karroo occur in the foreland of the great 

 east-west belt of compression, part of which forms the southern margin of Africa. In this fore- 

 land subsidence had continued since Lower Devonian times up into the Rhaetic. He sug- 

 gests that consequent on the sagging of the crust, or else causal thereto, a huge volume of sub- 

 crustal matter was bodily transferred outwards and upwards into the undisturbed regions 

 beyond the zone of corrugation, penetrating the strata and leading to their upheaval. The 

 injected magma in the foreland, and the effusions arising therefrom, represent a fraction only 

 of that squeezed forward in advance of the folding. The order of intrusion from above down- 

 ward is explained as the result of the strengthening of the crust by the early extrusions and 

 intrusions of shallow depth, and the consequent necessity for the magma, rising later, to find a 

 path of least resistance in the fissile sediments below these intrusions. On the assumption that 

 a submerged zone of folding, possibly the continuation of that south of the Karroo, lies between 

 Tasmania and Antarctica (an assumption made also by Kober '21 on other grounds). Du 

 Toit would explain similarly the origin of the dolerite-sills in these two localities, and also applies 

 the hypothesis to the explanation of the dolerite-sills of Brazil and New Jersey, and the develop- 

 ment of the Jurassic Rajmahal basalts of India. 



