39 



REGARDING THE COMPOSITION AND EXTENT OF 

 CERTAIN TERTIARY BEDS IN AND AROUND 

 LAUNCESTON. 



By R. M. Johnston. 



(Read 12th August, 1873.) 



GRAVEL. 



In the immediate vicinity of Launceston, and scattered over 

 the westward plains, are to be seen vast accumulations of 

 water-worn gravel, lying here and there in pockets, but 

 principally arranged in horizontal layers from 1 to 3 feet 

 thick, and associated with clays and tuffs, more or less 

 laminated. 



Some of these gravel beds were extensively exposed for the 

 purpose of obtaining ballast for the Launceston and Western 

 Railway, and I was thus enabled, during my leisure hours, to 

 glean some information regarding their contents. 



The most extensively exposed deposit is spread over that 

 elevated plain situated between Perth and Longford. This 

 plateau is 115 feet above the present channel of the South 

 Esk at Longford, or about 630 feet above the level of the sea. 



The contents of these beds are principally composed of 

 silicious pebbles and gritty concretions. The pebbles are all 

 more or less rounded and waterworn, but the most remarkable 

 feature is that their surfaces present the appearance of having 

 undergone great alteration by heat subsequently. Frequently 

 adhering to the smooth surface of these pebbles, are angular 

 gritty concretions, which generally fracture before yielding at 

 point of contact ;j when the gritty mass parts at point of 

 contact with the pebble, it usually leaves a red burnt-like scar. 



OPALIZED WOOD. 



In great abundance and scattered throughout the whole 

 extent of these gravelly accumulations. At altitudes varying 

 from 50 to 700 feet above the level of the sea, are to be found 

 the silicified fragments of fossilized trees. These fragments, 

 though preserving some degree of angularity, are all water- 

 worn, and also give evidence of having been subjected to sub- 

 sequent heat. 



The internal structure of these trees is in various stages 

 of preservation ; generally, the minute cellular network, inter- 

 lacing the delicate medullary rays, is distinctly visible, and by 

 the aid of a good lens, the structure exhibits great beauty. 



In sections of smaller branches now in my possession, the 

 medullary rays radiate directly to the circumference in a 

 straight line. {8ee Fig. 24, a, 5, c.) 



