Age of the Alkali Rocks. 159 



hy small quartz veins, which, as Mr. Twelvetrees has described, 

 have shed a fair quantity of gold, since recovered in alluvial work- 

 ings near by. 



The Porphyry Dyke Cutting the Diabase at Kettering. 



While the age of the alkali series is thus proved to be post Lower 

 Permo-Carboniferous, its relations to the only other rock of the 

 district, the diabase, have hitlierto remained obscure. A ridge of 

 diabase runs down the eastern side of the peninsula formed between 

 the D'Entrecasteaui Channel, Port Cygnet and the Huon River, and 

 on the western part of the peninsula Permo-Carboniferous rocks 

 outcrop at the surface. Although Mr. Twelvetrees and other 

 geologists have made several traverses across this ridge separating 

 the S.W. and the N.E. occurrences of alkali rocks, no members 

 of the alkali series have yet been found in any part of this inter- 

 vening ridge. Impressed by this negative observation Mr. Twelve- 

 trees has explained it on the supposition that the alkali rocks are 

 .older than the diabase. 



In the latter part of bur visit Mr. Twelvetrees and I worked 

 northwards from the alkali outcrops at Woodbridge on the north 

 edge of Peppermint Bay, past the Permo-Carboniferous glacial 

 deposits of Little Peppermint Bay to Little Oyster Cove at Kettering. 



On the south side of Little Oyster Cove, going east for about 

 150 yards beyond the jetty, we found the diabase came down to the 

 shore. At this point a low outcrop, a few feet in height, occurs. 

 and an abrupt charge from diabase to alkali porphyry was 

 observed. The porphyry extends for 15 to 20 feet, and then just 

 as abruptly diabase comes in again, and remains nearly to the 

 east extremity of the bay, which is occupied by Permo-Carbon- 

 iferous sediments, while on rounding the point to the south diabase 

 comes in again. 



There is no doubt in my mind that the occurrence of Porphyry 

 with parallel walls and in abrupt contact with the diabase repre- 

 sents an intrusion of porphyry into the diabase. The only alter- 

 native explanation of the relations of the two rocks that occurs to 

 me is that of a large mass of porphyry detached by and included in 

 the diabase. I reject the latter explanation, and adhere to the 

 view that the porphyry is part of a dyke for two reasons. The 

 first is that although the exposure is limited, one can see that the 

 walls in contact with the diabase are parallel as one would expect 

 to find in a dvke. The second reason is that at the contact with 



