146 THE GEOLOGICAL AGE OF VOLCANIC ACTIVITY 



Jura age. Yet the Tambourine rock rests uncomfortably 

 on beds low in the system, as well as on the underlying 

 schist. 



The unconformability is interesting, for it shows 

 elevation during Trias- Jura times ; it also serves to 

 accentuate the fact that the mere superposition of lava 

 on portion of the system is no evidence, the horizon being 

 unknown, that the lava is of post Trias-Jura age. 



Should contemporaneous volcanic rocks prove of 

 wide extension, their study is likely to be of great assistance 

 to the stratigraphy of a system, that otherwise appears 

 to offer few sharply- defined limits on which to base the 

 divisions. The study may thus be of no small aid in the 

 intelligent prospecting of the coal measures. 



In those districts where volcanic rocks of both ages 

 occur, or where the age has not been definitely determined, 

 it will be necessary to preserve an entirely open mind 

 regarding the age of any particular rock met with, remem- 

 bering always that neither the contemporaneous nor the 

 later rocks necessarily belong to only one brief period of 

 vulcanicity. We know that elsewhere in Australia there 

 were two periods of volcanic activity in Tertiary times, 

 and it is not unlikely that representatives of both these 

 occur in our area. 



Volcanic materials, chiefly tuffs of no considerable 

 development, occur in Mesozoic rocks elsewhere in Australia, 

 but volcanic activity is considered to have been the excep- 

 tion, in Mesozoic times, throughout the world, a fact 

 conferring greater interest on such rocks as do belong to 

 that period. Where, as seems likely in this present case, 

 the volcanic activity was extensive, the interest is consider- 

 ably increased. 



It is hoped that these few notes, in drawing the atten- 

 tion of members of this Society to the question will help 

 towards its study by reducing the greatest difficulty — the 

 lack of observers where so much is to be observed. 



BIBLIOGRAPHY. 



Andrews, E. C. — 



An Outline of the Tertiary History of New England. 



A Preliminary Note on the Structure of Mt. Lindsay. 



Records Geol. Surv., N.S.W., Vol. VII., Part III. 



