BY LOFTUS HILLS, M.B.E., M.SC. 119 



worked out in even approximate detail, this must be regarded 

 — as the author of it himself regarded it — as a tentative 

 approximation. 



(2). Cambro-Ordovician. 



It is in this system and in the succeeding Silurian sys- 

 tem that the greatest progress in Tasmanian stratigraphy 

 has been accomplished during the period under review. 



In 1902 the late T. S. Hall described some graptolites 

 from the slates in the Dundas district, and determined them 

 as being Ordovician types. Unfortunately, however, there 

 is some doubt in regard to the reliability of this determina- 

 tion in fixing the age of the Dundas slates, for repeated 

 search, both at the locality whence Hall's specimens were 

 supposed to have been procured and elsewhere in the series, 

 has signally failed to provide another specimen. Undoubted 

 Ordovician graptolites have been found in the Permo-Car- 

 boniferous glacial till at Wynyard, but with the above ex- 

 ception of the late T. S. Hall's specimens no graptolites have 

 been discovered in situ in Tasmania, in spite of diligent 

 search. 



The Dundas slate and breccia series, of which the typical 

 rock-type is a finely fissile purple slate, underlie with a 

 marked unconformity the slates and sandstones definitely 

 determined as Silurian. They are similarly definitely estab- 

 lished as unconformably overlying the mica-schists to which 

 a Pre-Cambrian age has been ascribed. The series then is 

 either Cambrian or Ordovician in age, and owing to the 

 failure to obtain information of the Ordovician determination 

 by the discovery of further graptolites, it is at present pre- 

 ferred to refer to them as of Cambro-Ordovician age. 



To this dual system are also referred the following 

 rock series named after the localities in which the chief de- 

 velopment of each occurs: — 



Read-Rosebery Schists. 



Balfour Slates and Sandstones. 



Mathinna Slates and Sandstones. 

 These three series have been described in some detail. 

 The Read-Rosebery schists have been dealt with somewhat 

 fully by the writer in Bulletins 19 and 23 of the Geological 

 Survey. These schists, the origin of which was previously 

 very obscure, have been now demonstrated to have been 

 mainly sedimentary in origin, pyroclastic material constitut- 

 ing what is not purely sedimentary. Their structural fea- 



