2 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. [Memoie "v A l.xix; 



the igneous rocks themselves. Perhaps the earliest hint of the possibility of such a connection 

 is found in Le Verrier's suggestion ('88) that basic rocks were developed at periods of terrestrial 

 calm, and acid rocks in times of great crust-disturbance. Harker ('96), followed by Prior ('03), 

 Becke ('03), and later by Suess ('09), held that in periods and regions in which the crust was 

 in a state of tension, producing block-faulting, rocks of a generally alkaline facies were produced, 

 while in regions of compression and folding the much more abundant rocks of the gabbro, 

 diorite, and granite series were produced. 



Stein mann ('05) urged that "under the depths of the sea magmas of extreme basicity 

 collect and are injected or poured out during the folding of the abyssal regions, while in the 

 basements of the continents and under shallow seas acid magmas are brought forth * * *. 

 In wearisome uniformity appear serpentine (and peridotite) and gabbro, generally in stocklike 

 masses, spilite, variolite, certain diabases and diabase-porphyrites, as dike-forming rocks in 

 certain zones of the younger folded mountains, while a typical effusive facies is seldom seen." 

 The conditions of injection of the basic rocks he thought to be essentially different from that 

 of the granitic and dioritic rocks. It is the alpine (overthrusting) type of injection as distinct 

 from the cordilleran. Instances are cited of intrusions of the alpine type, in Ordovician, 

 Carboniferous, and Mesozoic times in many regions. 



Sacco ('05) attempted to show that the Cretaceous period was the epoch of greatest 

 development of ultrabasic and basic rocks, but he considered together a number of rock types 

 that do not seem to be genetically closely related to one another, either tectonically or petro- 

 graphically. 



Daly ('06, '14) introduced the conception of abyssal injection and assimilation as a cause 

 of orogenic movement, and the consequent injection into higher portions of the crust or extru- 

 sion of magma. In this view the bulk of extrusive rocks consist of more or less undifferentiated 

 basalt, the primitive magma, while the intrusive masses have assimilated the invaded and 

 more acid crust-materials, and have become gravitationally differentiated. In his later state- 

 ment the summary given of the field-occurrence of the basic and ultrabasic rocks emphasizes 

 the importance of the gravitative separation, so that instances in which it may possibly be 

 exemplified have a prominence above the greater number of occurrences where no such separa- 

 tion seems apparent. The reason for such partiality in statement is, however, clear from the 

 very interesting and stimulative discussion given. 



Suess's ('09) generalization concerning the green-rocks has been quoted above. The intru- 

 sion is held to have taken place during a period of movement and into planes of differential 

 movement, or of bedding in the invaded rock. Thus, is explained the very frequent occurrence 

 of peridotite or serpentine in long, narrow, sill-like masses parallel to the general strike of the 

 structure lines of the surrounding region. A group of rocks have been classed by Suess under 

 the single name of the green-rocks ("pietre verdi") which have rather diverse characters and must 

 be specially considered. 



Dewey and Flett ('11) have urged that a special group of rocks should be recognized, the 

 spilitic suite, which range from picrites and dolerites to quartz-keratophyres, and are character- 

 ized by the abundance of albite. These were erupted as flows or sills in offshore regions in 

 which crust-subsidence was proceeding unaccompanied by folding and faulting. They are thus 

 considered to be distinct from Steinmann's ophiolitic rocks in their mode of origin. 



Harker ('11) has accepted this group (omitting the quartz-dolerites) as a special sub- 

 division of the alkaline branch of the igneous rocks both on tectonic and petrographic grounds, 

 but Daly ('14), carrying to a further application a conception of Bowen's ('10), believes that 

 their high content of albite is merely the result of the concentration of soda in the upper portion 

 of an intrusive magma, by resurgent water derived from the wet sediments it has invaded, 

 and he sees nothing especially alkaline about the rock-masses as a whole. They are merely 

 flows and shallow intrusions formed during the sedimentation in a subsiding geosyncline, that 

 preceded the main orogenic movement and plutonic intrusions. 



C. H. Smyth ('13) also invoked gaseous transfer (but as the result of the action of juvenile, 

 not resurgent, gases, chiefly water) by which alkalies and the elements especially present in 



