144 PROCEEDINGS OF THE BOSTON MEETING 



of gray schist and intrusive granite that it is diflacult to map them sepa- 

 rately ; yet there is no evidence of marked interaction between the gray schist 

 and granite. Lawson has mapped this gray schist as Coutchiching along the 

 Minnesota coast and believes that it is of sedimentary origin, chiefly because 

 of its banded character. The writer's statements apply only to Lawson's 

 Coutchiching of Minnesota and not to his Coutchiching in general. 



The banding is generally faint, being due to a slight concentration of small 

 brown biotite flakes. Rarely it consists of seams rich In brown biotite up to 

 one-half inch thick. Sometimes the schistosity of these bands is diagonal to 

 the banding — a trait which would seem to indicate that the banding antecedes 

 the schistosity. 



One of the outstanding characteristics of the rock is its uniformity of grain, 

 mineral composition, and general field appearance. The mineral constituents 

 are uniformly brown biotite, orthoclase, oligoclase, and quartz, the schistosity 

 being due mainly to the parallelism of the mica. The concentration of the 

 mica in parallel laminre is the cause of both the faint and prominent gneissic 

 structure. The ratio of feldspar to quartz is about the same as in a typical 

 granite. 



The writer believes that this rock is of igneous origin, because of its uni- 

 formity over great areas and because its feldspars conform rigidly to the law 

 of association characteristic of the feldspars of igneous rocks. It is sug- 

 gested that the banding, where prominent, may be due to an original tuffaceous 

 or to a rhyolitic flow strucure. Some of the samples taken from this forma- 

 tion near the contact of the Keewatin greenstones on Rainy Lake are un- 

 doubted rhyolites. The fineness of grain of the schists as a whole suggests 

 that originally they were rhyolitic, glassy, felsitic, tuffaceous rocks. 



Presented by title in the absence of the author. 



TECTONIC CONDITIONS ACCOMPANYING INTRUSION OF BASIC AND VLTBA- 



BASIC IGNEOUS ROCKS 



BY W. N. BENSON 



{Abstract) 



The investigation of the great serpentine belt of New South Wales revealed 

 to the writer a mass of ultrabasic rock extending intermittently for over tw(^ 

 hundred miles, with a breadth of generally only a small fraction of a mile. 

 It occupied a well marked zone of faulting and its intrusion was clearly de- 

 pendent on the movements which determined also the principal structures of 

 the region. In the consideration of this relationship the writer determined 

 to put to an independent test Suess' generalization, that the "green rocks form 

 sills in dislocated mountains that sometimes follow the bedding planes and 

 sometimes the planes of movement." For this purpose he has made a study 

 of the geological history of every region containing ultrabasic rocks through- 

 out the world the literature of which was accessible to him. He has also 

 had the opportunity of examining on the field numerous occurrences in Great 

 Britain, in Europe, and in New Zealand, of examining the petrological collec- 



