6 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. ^"""'"vol-xix 1 ; 



In considering these two groups we must carefully distinguish between two types of ultra- 

 basic rocks. Normally the more abundant are the earliest ultrabasic and basic intrusions of a 

 complex, the subordinate type are the petrographically very simdar dikes commonly termed 

 "picrite," 3 and these form the latest differentiates in the "phase of minor intrusions," which 

 follows that of the major intrusions in a cordilleran complex. Harker ('04) has emphasized 

 this distinction in Skye. The well-known hornblende-peridotite of Schriesheim is a further 

 example of these intrusions. Analogous to them, though not so basic, are the dolerite-dikes which 

 form the final products of the ' ' phase of minor intrusions ' ' in many normal cordilleran complexes. 

 No petrographic distinction can generally be drawn between the basic rocks of the major intru- 

 sions of the laccomorphic and cordilleran type, in spite of the apparent difference in the extent 

 to which they have been subjected to lateral pressure. 



Some of the members of the cordilleran group, particularly the Ivrea rocks (see p. 26) 

 were included among the "green-rocks" in Suess's generalization. 



In the third group of occurrences, which we may term the alpine type, are comprised the 

 majority of the "green-rocks" as considered by Suess. These are also the "ophiolitic rocks" 

 as defined by Steinmann ('05), in which serpentines and gabbros are intimately associated with 

 amphibolites" and diabases, sometimes showing the ellipsoidal form of pillow lavas. They 

 occur in regions that have been intensely disturbed by overthrusting and alpine orogeny. The 

 conditions under which they may have been formed are far from being obvious. We must 

 leave this group aside for the present as one requiring further analysis and endeavor to discuss 

 it subsequently (p. 68). With the possible exception of certain of the diabasic members, the 

 chemical features of the rocks of this group are those of the more basic members of the cor- 

 dilleran group. 



In addition to these, there is a fourth mode of occurrence of ultrabasic and basic rocks, 

 namely, in the form of lenticular masses generally conforming to the structure lines or schis- 

 tosity of the gneissic, or schistose complexes in which they occur. It is not always possible in 

 such cases to state the conditions under which these masses were injected, though it is not 

 improbable that many are essentially intrusions of the alpine or cordilleran types and were 

 erupted during orogenic movement. The ultrabasic and basic igneous rocks of the Caledonian 

 series in Norway afford good examples. 



A very uniform group comprises the sills of dolerite, and especially quartz-dolerite, which 

 invade approximately horizontal but not folded strata, underlying very broad areas, but some- 

 times rising obliquely through the invaded strata. Rarely they are associated with apparently 

 indubitable examples of gravitationally differentiated laccolites. The mechanism of their 

 intrusion is somewhat obscure, but its action must have been rapid to permit the extension 

 without chdling of such widespread but narrow intrusive sheets. It is evident that it was not 

 accompanied by marked lateral pressure. The dolerites — and the associated laccolitic masses 

 with basal differentiates when these are present as in Natal (p. 49) — have much the same range 

 of chemical characters as occur in basic members of the cordilleran group of rocks. The Whin 

 Sill of England, the Palisades on the Hudson River, the dolerites of the Karroo, of Tasmania, 

 and of Antarctica are typical members of this group. The average composition of these rocks 

 has probably undergone comparatively little differentiation from the parent magma though 

 relative differentiation may be present. 



The occurrences of basic intrusive rocks with alkaline features may be classed in two 

 further divisions, the first of which being that specially considered by Becke ('03) and typified 

 by the Tertiary complex of the Bohemian Mittelgebirge and the late Palaeozoic rocks of 

 Scotland, (Tyrrell '12). We may term them the alkaline plateau-group, thus distinguishing 

 them from those last considered. They comprise rocks which were not classed under the green- 

 rocks by Suess, the essexites, theralites, teschenites and picrites in the original sense of the term. 



1 The inclusion of these rocks under the term "picrite" is an extension of the significance given to it by the originator Tschermak ('67) who 

 applied it to the melanocratic phase of teschenites, formed under completely different conditions from the rocks considered here (see p. 7). This 

 usage, therefore, seems disadvantageous in many respects and has been abandoned by Dr. Harker. See also Bailey's discussion ('10). 



