256 STRATIGRAPHY 



close microscopic similarity between quartz dolerites of the Antarctic and those of 

 Tasmania. As regards geological age the Tasmanian dolerite sills have strongly 

 intruded the Trias-Jura rocks, and are themselves intruded by the Tertiary basalts. 

 Moreover, in the neighbourhood of Launceston their surface has been eroded to con- 

 siderable depths and then covered by the rocks of the Windmill series, which are 

 probably of Eocene or at all events Early Tertiary age. Thus the Tasmanian dia- 

 base intrusion takes place somewhere between early Tertiary and late Jurassic time. 

 Its date of intrusion is therefore possibly Cretaceous. In South Africa similar 

 dolerite sills were intruded into the rocks of the Karoo series either in late Jurassic, 

 or possibly in lower Cretaceous time. In British Guiana the quartz dolerites, 

 occurring there as dykes and sills, are considered to be intruded in Mesozoic time. 



Mr. Benson compares also with the Antarctic dolerites the quartz dolerites near 

 New York forming the famous palisades on the Hudson. These intrude the Newark 

 system, perhaps of Trias-Jura age. 



Dr. G. T. Prior has shown in his account of the quartz dolerites of Natal and 

 Zululand that they are strikingly similar to those of the palisades and of the 

 Antarctic. 



There is here a strong probability of the Antarctic quartz dolerites and those 

 of Tasmania forming part of one great tectonic hinge. If, as seems probable, the 

 Tasmanian dolerites were intruded in Cretaceous time, the age of the Antarctic 

 dolerites would be also Cretaceous. They serve, therefore, to fix the upward limit for 

 the date of the Beacon Sandstone. Thus the coniferous wood practically limits it 

 downwards to Carboniferous, while the quartz dolerite places its upward limit below 

 the Cretaceous. 



