VOLCANIC NECKS OF HORNSBY AND DUNDAS. 511 



invariably secondary. It occurs always in cracks which 

 would form solution passages and nearly always is in 

 association with calcite. 



Spinels. — Pleonastc is present in many of the rocks. It 

 is bluish-green in colour and without cleavage. It is, how- 

 ever, crossed by large cracks, in which in the more altered 

 rocks, opaque oxides may be deposited. In most instances 

 it forms rounded or irregularly bounded grains, but in many 

 cases it forms a granophyrie intergrowth with the pyroxene. 

 This will be described in more detail later. 



Picotite occurs in the ultrabasic rocks. It forms dark 

 brown to lighter brown-green rounded grains, without 

 cleavage, but with cracks often containing black oxides. 

 There is no sign of its forming graphic intergrowths with 

 the pyroxene. Professor David and Messrs. Watt and 

 Smeeth 1 reported the presence of chromite at Dundas. 

 While this may occur, in the two ultrabasic rocks I have 

 analysed, the spinel is clearly picotite as shown by the 

 high percentage of alumina. It therefore seems probable 

 that the majority, if not all, of the brown spinel is picotite. 

 According to Rosenbusch, chromite is indistinguishable 

 from picotite except by a chemical analysis which was not 

 made by the authors cited. 



Apatite has not been noted. 



Carbonates, calcite, siderite, and magnesite occur in 

 irregular grains, rarely with any crystal outline ; also in 

 very fine dustlike particles in the intergranular planes and 

 in cracks. 



Rocks of the Plutonic Inclusions. 



The mineralogical composition of the rocks maybe shown 

 in the following manner. Basic and ultrabasic plutonic 

 rocks of the normal type, composed essentially of relatively 



1 Proc. Roy. Soc, 1893, p. 401. 



