Chapter II. 



A PROVISIONAL TECTONICO-PETROGRAPHIC CLASSIFICATION OF 

 INTRUSIVE COMPLEXES OF RASIC IGNEOUS ROCKS. 



From the various considerations already adduced it will be quite clear that no general 

 philosophy of petrogenesis should be based upon a study of a portion of the igneous rocks alone, 

 however complete it might be in regard to geological, petrographical, and physico-chemical 

 circumstances; and therefore, since no study of the intermediate and acid types of normal and 

 alkaline igneous rocks has been made by the writer comparable in detail with those upon basic 

 igneous rocks here given, it must be recognized that the generalizations which follow are derived 

 perhaps too exclusively from the study of basic rocks. If, however, they are found to be an 

 adequate statement of the modes of occurrence of such rocks, they may then be combined with 

 the results of similar studies upon the other types of rocks into a general petrogenetic theory. 



While no invariable rules for mode of occurrence of basic intrusive rocks seem yet dis- 

 coverable, there emerge from the studies which follow the conception of several modes of occur- 

 rence, so frequently represented that they seem to result from fundamental principles of 

 petrogenesis, while the exceptions to these are especially worthy of study, since in all probability 

 they will throw most light upon the nature of the controlling conditions and the relationships 

 to one another of apparently distinct modes of occurrence. 



A frequent association of basic intrusive rocks is in a series of masses ranging from perido- 

 tites to granites, the relative size of the individual masses as mapped being often an inverse 

 function of their basicity. In these there may be a gradual variation between the apparently 

 laccolitic type of complex to that of a sill-and-batholitic phase. In the former the basic rocks 

 appear as more or less of a marginal zone, and are commonly supposed to indicate either gravi- 

 tational differentiation in situ, productive of a stratiform arrangement, the strata merging more 

 or less completely into one another, or they may result from differentiation as a result of crys- 

 tallization with active diffusion in the magma in the manner suggested by Harker's ('94) 

 study of Carrock Fell, or the alternative explanation, viz, that as a result of chilling the margin 

 crystallized without much differentiation, but the central portion consolidated only after much 

 gravitative differentiation (cf. Daly '14, p. 358). In the sill-and-batholitic complexes the several 

 portions of the complexes have been separated, and more or less stratiform or sill-like masses of 

 peridotites and gabbros were formed, which were invaded subsequently by less regular batholiths 

 of diorite, granite, etc., of which, however, the longer axes may still be parallel to the structure 

 lines of the country. The complexes of the Bushveld ' and of the Southern Urals may illustrate 

 the former type; those of Garabal Hill and the Harz Mountains tend rather to the latter type. 

 While the latter are always formed during a period of great lateral pressure, this does not appear 

 to have been very noticeable during the formation of the first type of complex. Complexes of the 

 latter type we may therefore conveniently class as of the cordilleran type (cf. Steinmann's usage 

 of the term) ; for the former the phrase laccomorphic type may perhaps be suggested, though the 

 term lopolith has been applied by Grout ('18) to certain instances of this type, notably the Duluth 

 complex. 2 Between the two extremes there are a number of intermediate forms, as may be in- 

 ferred from Bowen's comment ('15) on the varying extent of lateral separation of the fractions of 

 a gravitationally differentiated complex. 



■ The work of Brouwer and Humphrey now permits us to doubt the relationship of the Pilandsberg nepheline rocks with the Bushveld 

 complex, though it seems still just possible that the two may be genetically connected. 



a The convenient term "stromatolith" has been preoccupied for a very different significance (Foye '16) and for the name of a sedimentary 

 rock (Kalkowsky). 



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