188 TERTIARY ROCKS OF AUSTRALASIA, 



Johnston (R. M.) Reference List of the Tertiary Fossils 

 of Tasmania. Papers and Proc. Hoy. Soc. of Tas. 

 for 1886, pp. 124-140. 



Lacustrine Formations. — The more important lacus- 



tnne formations, as might be expected, are mainly found 

 in the original valleys and eroded basins of the earlier 

 rocks, and generally consist of regular or irregular bands 

 or layers of white, grey, or ferruginous sandstones, alter- 

 nating with grits ; blue, Mdiite, yellow, or blackish clays ; 

 hgnites ; and sometimes, in the neighbourhood of the older 

 slates and crystalline rocks, the ancient channels formed in 

 them contain drifts of a richly auriferous or stanniferous 

 character. Many of the formations are found along the 

 course of existing rivers and watercourses in the form of 

 raised bordering terraces. In other places, as in the Laun- 

 ceston Tertiary Easin, they occupy the floor of broad 

 undulating plains, covering an area of not less than 600 

 square miles, and ranging from 400 to 1000 feet in thick- 

 ness. Being comparatively of a loose and incoherent 

 nature, the beds are unable to resist the eroding influences 

 of air and v>^ater, and are, therefore, greatly denuded along 

 the course of existing rivers and their tributaries. The 

 extent of this denudation is well exemplified along the 

 lower course of the North Esk in the neighbourhood of 

 Breadalbane, St. Leonard's, and Launceston. In this 

 vicinity it is estimated that strata from 15 to 20 miles 

 long, by 1 to U miles broad, have been denuded to a 

 depth ranging from 50 to 500 feet. These lacustrine 

 deposits are found throughout the island from sea level to 

 an altitude of four thousand feet above it ; sometimes, as 

 at Magnet Range, Mount Bischoff, Branxholm, and 

 Ringarooma, concealing and composed of the waste of the 

 oldest or pre-Archaean rocks, with associated stanniferous 

 granites and porphyries ; again, as at Macquarie Harbour, 

 Beaconsfield, Lefroy, Back Creek, Tullochgorum, Man- 

 gana, and Black Boy, bordering and concealing the 

 Bilurian slates with quartz dykes and veins, from the 

 destruction of which they have derived their auriferous 

 drifts. At Geilston, Cornelian Bay, Hobart, Sandy Bay, 

 and One-Tree Point they are associated with the mud- 

 stones and intrusive greenstones ; while at Launceston, 

 Longford, Ross, Jerusalem, and Hamilton, they are mainly 



