TITLES A^•D ABSTRACTS OF PAPERS 145 



tioiis of many workers on these rocks, and of personal discussion with them 

 of the problems involved. The work became enlarged during its progress by 

 the addition to it of the consideration of rocks classed in the spilitic suite by 

 Messrs. Flett and Dewey and other rock types not considered by Suess among 

 the green rocks. 



Considering all these occurrences, there appear to be certain types of condi- 

 tions of development so frequently repeated that they seem to correspond to 

 fundamental processes in the development of igneous rocks. We may pro- 

 visionally classify these as follows: 



1. The complexes of a generally laccolitic form (sometimes lopolithic) 

 represented by the Duluth, Sudbury, and Bushveld complexes. The extended 

 studies of the last of these seem to render difficult of application the view 

 that they result simply from the gravitational differentiation of a magma 

 intruded at a single act, and to favor instead the view that they result from 

 a succession of intrusions of increasing acidity, extending over a long period 

 of time. These are developed where the lateral pressure is not great and the 

 successive fractions invaded are not geographically separated from one 

 another to any great extent. 



2. In the Cordilleras, where lateral pressure is more noteworthy, the separa- 

 tion of the successive fractions is a marked feature, and the more usually 

 concordant character of the basic and ultrabasic intrusions, as contrasted 

 with the strongly transgressive granitic intrusions, is clear. The microscopic 

 strucures of the basic and ultrabasic portions of such complexes do not seem 

 to lend support to the suggestion that the rocks did not crystallize from a 

 magma of the composition of the rock which they now form, but were a col- 

 lection of sunken crystals, with just sufficient magma mingled with them to 

 allow of viscous flow. Rather they seem to be successive intrusions of 

 diverse magmas, the differentiation having occurred, not in the upper portion 

 of the crust, but in some deep-seated reservoir below. The phenomena of 

 hybrid rocks occur between closely succeeding intrusions of diverse com- 

 position. 



3. In Alpine regions, where the effects of lateral thrust are very great 

 indeed, the sill form becomes most marked in the more basic intrusions, which 

 may occur in the forward thrust portion of great overthrust foldings. Here 

 occur what Steinmann has termed ophiolitic rocks, but the writer does not 

 concur with Steinmann in his conception of them. After a detailed considera- 

 tion, he is of the opinion that they consist in part of flows and shallow intru- 

 sions of a somewhat spilitic nature in the folded strata, and in part of in- 

 trusions that occurred during the subsequent period of overthrust, and were 

 injected in large measure along the plane of overthrusting, and may indeed 

 have locally extruded from the front of the thrust-plane, to be subsequently 

 overridden by the advancing crust-flake, under which the continued flow of 

 magma produced coarse-grained intrusive rocks. In the absence of detailed 

 information concerning the peculiar association of serpentines, gabbros, 

 basalts, and agglomerates in the Cretaceous folded rocks that surround the 

 massif of peninsular India, it is impossible at present to state the relationship 

 of these intrusions to tlio green rocks of the more folded Alpine regions. The 



X— Bill. Okol. Soc. Am., Vol. 31, 1019 



