100 PROCEEDINGS OF THE SAIXT LOUIS MEETING 



that it is difficult to imagine these rocks to be acid differentiates of a gab- 

 broitic magma, since gabbros are present oulj' in insignificant patches in the 

 district. Moreover, the syenites are not at all like the granophyric rocks asso- 

 ciated with gabbros in Minnesota and at Sudbury. 



Dr. F. F. Grout : It may be well to record two points in which the Dulutli 

 gabbro and related sills are less favorable to Doctor Bowen's idea of anortho- 

 site formation than he thinks. At Dulutli gabbro was intruded at two periods. 

 The first and smaller mass seems to contain about 85 per cent plagioclase. It 

 has large volumes of anorthosite differentiate. The second intrusion has more 

 nearly 70 per cent plagioclase, but has numerous layers of anorthosite of 

 smaller size. Since magmas vary from 70 to 85 per cent in plagioclase, as 

 intruded, it seems certain that some may be intruded with over 90 per cent 

 plagioclase when they would be classed as anorthosite. Bowen, in outlining 

 the origin of anorthosites, refers to Winchell's observation that some diabases 

 have lumps of anorthosite and scattered crystals of plagioclase. I have seen 

 the lumps described, and have seen lumps apparently identical in nature and 

 relation in the "red rock," from which they could not have formed by segrega- 

 tion. I have no hesitation in stating that these lumps of anorthosite are 

 xenoliths, not segregations. 



Remarks were also made b}^ Professors James F.Kemp and Frank D. 

 Adams. 



PETROLOGY OF RUTILE-BEARING ROCKS 

 BY THOMAS LEONARD WATSON 



(Al)stract) 



The paper presents a discussion based on chemical and microscopic studies 

 of the petrology of rutile-bearing igneous rocks in general, including the more 

 important districts in this country and abroad. 



Read by title in the absence of the author. 



INTERNAL STRUCTURES OF IGNEOUS ROCKS 

 BY FRANK F. GROUT ^ 



{Al)stract) 



The igneous rock structure emphasized is an alternation of bands of vary- 

 ing mineral composition. In many places this is related to a fluxion structure 

 and a sheet structure. Many references to descriptions of one or several such 

 structures are cited. The Duluth gabbro shows all three structures conspicu- 

 ously. 



In nearly all of the many references it is stated that the structure in the 

 igneous rock shows a general parallelism to the contacts of the mass — that is, 

 the structure developed under some control by its walls. There are so few 

 exceptions that the structure may be safely used as a guide to form. 



The origin of the structure has been discussed by several men. Eliminating 

 those cases in which metamorphic banding or half-fused layers are indicated, 

 the Suggestions are : (1) Partial assimilation, (2) lit par lit. (3) deformation 



1 Introduced by W. H. Emmons. 



