48 ox MBSOZOIC DOLBRITB AND DIABASE. 



near Hobart have summits composed of this rock. From' 

 the eastern side of the Central Tiers it is continued to the 

 Eastern Tiers and the hillj- ground from Swansea north- - 

 wards to St. Mary's. Ben Lomond, the Mount Nicholas 

 Range, Tower Hill, Mount Victoria, and Mount Saddle- 

 back are north-east out-liers fringed with Permo-Car- 

 boniferous and Jura-Trias. The northern extension passes 

 under the sands and clays of the Launceston Tertiary basin,, 

 and re-appears along both sides of the River Tamar. It 

 extends to Mounts Barrow and Arthur and Ben Nevis. 

 There is an isolated patch of it at the mouth of the Mersey 

 and at Port Sorell. On the East Coast it abuts on a fringe 

 of granite on Maria Island, Schouten Island, and Frey- 

 cinet's Peninsula. 



A peculiar feature is the almost invariable association 

 with it of Permo-Carboniferous and Trias- Jura beds. The^ 

 whole periphery of the area forming the Central Tiers is 

 fringed with a narrow zone of these beds, and the same 

 holds good in the case of all the isolated peaks. How can 

 this association be explained ? It has been suggested that 

 dolerite capping has protected underlying sediments, and 

 that the latter do not merely hang on the flanks of the 

 igneous table-land, but actually lie beneath the eruptive 

 capping, as the lower formations would do in the case of a 

 sill. The few boring trials which have been made in 

 different parts of the Island lend no support to this 

 suggestion. They have been made through the Permo- 

 Carboniferous and Trias-Jura beds, and traversing these,, 

 have penetrated into the dolerite below. On the other 

 hand, no trial has been made of boring through the 

 dolerite at surface with a view of reaching the coal mea- 

 sures : as a matter of fact, we do not know whether the 

 rock on the Tiers is a denuded intrusive sheet, concealing 

 sedimentary^ rocks below it, or whether it is a vast 

 eruptive mass in situ. The thickness in other parts of the 

 island ma:kes it difficult to believe that it is an intrusive 

 sill. The thickest sill we can find mentioned in geological 

 literature is the sill of basic rock in the Shiant Isles, off 

 Scotland, described by Sir Archibald Geikie* as showing a 

 sea-wall 500 feet high. But even this surprising thickness 

 falls below the development of the massive rock which 

 occupies the upper part of Mount Wellington. At the 

 same time numerous minor intrusions in the form of 

 dykes penetrate the Permo-Carboniferous and Trias-Jura 

 sedimentary beds, so that we have two rather clear types 



* Qviarterly Jovirnal Geo. Soc, 1896, p. 375. 



