156 



The Occurrence and Origin of Certain Quartz- 

 Tourmaline Nodules in the Granite of Cape 



WlLLOUGHBY. 



By C. E. Tilley, B.Sc, A.I.C., Demonstrator in Geology and 

 Mineralogy, University of Sydney. 



(Communicated by Professor Walter Howchin.) 



[Read July 11, 1919.] 



Plates XXIII. and XXIV. 



CONTENTS. Page. 



I. Introduction ... ... ... ... ... 156 



II. General Description ... ... ... ... 156 



III. The Quartz-Tourmaline Nodules ... ... 157 



IV. Summary ... ... ... ... ... ... 164 



I. Introduction. 



The present paper is devoted to a short description of an 

 occurrence of some remarkable aggregates composed essenti- 

 ally of quartz and tourmaline with felspar, which are 

 developed in a mass of aplite intrusive into the granitic 

 headland of Cape Willoughby, Kangaroo Island. The paper 

 really forms part of a more extensive study of the petrology 

 of the Cape Willoughby granite and its allied intrusions. 

 The publication of these data is reserved for a later date. 



After a review of the occurrence, and reference to pre- 

 viously published descriptions of similar aggregates at other 

 localities, the probable mode of origin of the nodules is 

 outlined. 



II. General Description of the Occurrence. 



Cape Willoughby consists of a large mass of granite 

 intruded into a series of quartzites, quartz-mica-schists, and 

 mica-schists of probably Cambrian Age. The granite is an 

 even-grained rock consisting of quartz, microcline, plagioclase 

 (oligoclase-andesine), and biotite. Under the microscope the 

 accessories are seen to be muscovite, apatite, and zircon. 

 The most striking feature of the rock is the presence of sub- 

 idiomorphic crystals of quartz showing a remarkable blue 

 opalescence. This granite occupies an area of approximately 

 two square miles, and has a coastline length of about' five 

 miles. 



Into this granitic mass are intruded a highly interesting 

 series of aplitic and pegmatitic rocks which are obviously 



