The Heathcotian. 157 



IV. — The Petrography of the Heathcotk Rocks. 



With Mr. Hewitt's description of tlie petrography of the rocks 

 I am in agreement, except in a few minor matters of nomen- 

 clature. The rocks may be divided into four groups — the 

 unaltered sediments, metamorphic rocks, acid igneous rocks, and 

 basic igneous rocks. 



The unaltered sediments show nothing of special interest. 

 Reference has already been made to the occurrence of fragments 

 of the diabasic materials in the Silurians. The ordovician rocks 

 are normal clastic rocks, considerably crushed, but not foliated. 



The metamorphic rocks of the Heathcote series are of two 

 types. They include schists and sandstones. As a type of the 

 altered schists I may refer to a specimen collected 130 yards 

 west of the so-called copper mine, on the Murray road, two and 

 a-half miles north of Heathcote. The rock is a phyilite, traversed 

 by two distinct cleavage planes. The rock was Hrst altered by 

 the development of a foliation, which formed alternate bands of 

 fine grained argillaceous matter and quartz. It was subsequently 

 cleaved obliquely to the foliation planes. The rock breaks 

 readily along the second planes, and these are clearly shown in 

 section as they are marked by limonite stains. The typical 

 rock of this group is phyllitic schist. 



The second type of the metamorphic rocks includes a series of 

 black cherts, which are especially well shown near Lady's Pass 

 in Dargile. These cherts when freslj are of a black colour with 

 a satiny lustre. When exposed for some time to the weather, 

 they become grey or they may be quite bleached. These rocks 

 have been described by Mr. Howitt as adinoles. An adinole is 

 a rock altered by a basic intrusion, and the term seems to 

 imply the absorption by tlie altered rock of some material 

 from the intrusive rock. Two analyses of adinoles given I)y 

 Rosenbusch^ show 8.33 and 7.77 per cent, of soda respectively, 

 whereas the analysis of the Heathcote rocks, made l)y Mr. F. 

 Stone for Mr. Howitt,- show only 0.84 and 1.98 per cent, of 

 alkalies. The analyses are as follows : — 



1 Rosenbusch : Elemente der Gesteinslehie, 1898, p. 333. 



2 Howitt, op. cit., p. 8. 



