The Heathcotian. 159 



The plagioclase, according to Mr. Hewitt's measurements, is a 

 member of the labradorite group. There is also an araphibole. 

 ■either anthophyllite or a monoclinic species witli a nearly 

 straight extinction. The rocks of this second type Mr Hewitt 

 •described as later effusions from the aplite magma, and he calls 

 them labradorite-pophyrite. 



The two varieties of the rocks are probably associated, as Mr. 

 Howitt concluded, and the labradorite-porphyrite is intermediate 

 between the more acid variety and the diabase. 



The use of the term aplite for either of the rocks seems to me 

 open to doubt. Taking Rosenbusch's^ last definition of the term 

 aplite, he describes it as "a panidiomorphic granular rock, which 

 ■consists of predominant potash felspar (orthoclase, microcline 

 with lattice structure, microperthite, more rarely microclinmicro- 

 perthite), and quartz with acid plagioclase and a very limited 

 •quantity of muscovite and biotite, or both these micas." The 

 structure, therefore, of an aplite is panidiomorphic, whereas in 

 these Heathcote rocks the structure is hypidiomorphic. The 

 rock, moreover, occurs more in masses than dykes, and it .seems 

 to me safer to regard the rocks as fine-grained granodiorites. It 

 is quite natural to find them associated with later effusions, as 

 Mr. Howitt describes them, or possibly dykes having the chemical 

 character of rocks of the intermediate group. The porphyritic 

 granophyric labradorite rock in all-otment No. 16, at South 

 Heathcote, can be more easily explained as derived fioin a 

 granophyr than from an aplite. 



The typical granodiorites occur at the Red Hill, where tiiey 

 are seen to be clearly intrusive into tlie diabase series. They 

 also occur in the area round the water reserve, where they occur 

 as a series of small tors. 



The labradorite-porphyrite is best shown in South Heathcote, 

 ■on the northern side of Photograph Knob. 



The Diamase Sehies. 

 The rocks of this series, as Mr. Howitt has pointed out, are 

 very varied in their petrographic characters. Many of them, 

 such as the diorite, the enstatite-diabase, and most of the 



1 H. Rosenbusch : Eleiuente der Gesteinslehre, 1898, p. 206. 



