^0 E. 0. Teale: 



((II.) Purple and green tuff between Pen-Maen-Melyn and Pen- 

 .Maeii-Foel, South Wales, Cambrian, (Wilson), British Petrography, 

 Teall, page 223. 



The principal differences to be noted in comparing the Dolodrook 

 .tuff with the Cambrian one of South Wales are the greater percen- 

 tage of iron and smaller amount of lime and magnesia present in 

 the former* No other analyses have yet been made of similar rocks 

 of supposed Heathcotian age in Victoria. 



Thin sections of the fine grained tuff (No. 44), from the Dolo- 

 drook, show a banded and somewhat schistose' structure, with nume- 

 rous small sub-prismatic and irregular grains of basic plagioclase 

 felspar, many of which are orientated with their longer axes paral- 

 lel with the planes of schistosity, but many also lie at any angle, 

 frequently across the planes of foliation. Much chloritic and some 

 serpentinous material is present between the felspar grains, but 

 no fresh pyroxenes were distinguished. Magnetite is abundant, 

 and several small grains of quartz were noted. Ihe fragmental 

 structure, together with the chemical and miner alogical composition 

 -seems to indicate the nature of an altered subaqueous diabase 

 tuff. 



The secondary silicihcation, which is one of the marked features 

 of the diabase tuffs and allied rocks at Heathoote, is absent. 



Locality/ B. — The next section of importance is about two and 

 a half miles in a south-easterly direction, at the junction of Thiele's 

 Creek w^ith the Dolodrook. Here within a relatively small area 

 there are good exposures of the Serpentine, the basic sediments with 

 interbedded limestone, the Upper Ordovician graptolite bearing 

 slates; and the Silurian sandstones and shales (Map 2). 



Ihe actual contact of the Cambrian sediments and the Serpen- 

 tine is shown in the bed of the Dolodrook River, at Thiele's Creek 

 Junction. Here the sediments are of the nature of serpentinous 

 and chloritic conglomerates, grits and finer bands. 



A section of this is given by Mr. Dunn (16), who describes the 

 rocks as post-Ordovician, on account of the presence of dense 

 darker rock fragments, which he regarded as Ordovician slate, but 

 which microscopic evidence shows to be a basic igneous rock. The 

 fine dark bands described by Mr. Dunn as slate, prove to be tuffa- 

 coous, and similar in character with the basic sediments associated 

 with the Cambrian limestone in every case. 



In this vicinity there are two limestone outcrops associated with 

 the basic sediments. Additional thin sections of both the coarse 

 p.nd fine material indicate their xuffaceous character, and support 

 the vicAv also that they belong to the Garvey Gully Series. 



