160 Ernest W. Sheats .- 



the diabase the porphyry for about half-an-inch in width is quite 

 different in texture from the central .part of the mass. The small 

 porphyritic crystals are set in an exceedingly fine-grained paste or 

 ground-mass, much finer in texture than the normal ground- 

 mass, away from the contact. This feature I regard as a selvage 

 to the dyke produced by the rapid chilling of the intrusive mass 

 against the cold diabase walls. 



Kainozoic Age of the Porphyry Dyke at Kettering. 



The above evidence, I think, establishes the conclusion that the 

 prophyry at Kettering is a dyke, intrusive into the diabase, and 

 therefore younger than it. The age of the diabase sills in Tasmania 

 has been demonstrated to te post Upper Jurassic, since in several 

 places an intrusive contact with these sediments has been estab- 

 lished. It is generally believed to be probably Cretaceous in age,, 

 and to have been intruded during the earth movements, which led 

 to the breaking up of the Gondwanaland continental mass or 

 masses. 



If this view is correct the porphyry dyke at Kettering is probably 

 post-Cretaceous in age, and in that case belongs to some part of the 

 Kainozoic period. 



Relations of the Kettering Dyke to the other Alkali 

 Rocks of the District. 



While the field evidence as described above defines the age of the- 

 Kettering dyke as post-Diabase, and therefore almost certainly 

 Kainozoic, the field relations of the other alkali occurrences in the 

 district only enaVjle one to assert definitely that they are post-Lower 

 Permo-Carboniferous. 



We must turn to petrographic evidence to see whether or no there 

 are sufficient petrographical and mineralogical resemblances be- 

 tween the various members of the suite of rocks to make it probable 

 that they were all intruded during the same period. 



For this purpose I have had a number of sections cut of rocks 

 from the various localities mentioned above. This paper is not con- 

 cerned with the detailed microscopic characters of the rocks, and 

 my examination of the sections has simply been for the purpose of 

 correlation of the various dyke occurrences. 



Examination in the field or by hand specimens suggested that as 

 far as naked eye examination goes the rocks which most closely 

 resembled the Kettering dyke occur at Woodbridge and at Petchey's 



