134 Geology of Hobart Town. 



beds remains so constantly the same, and towards instead 

 of from an evidently upheaved rock ? Perhaps the local 

 dip, in the opposite direction on the sides of Mount Wel- 

 lington, may explain the difficulty. If a vast level plateau, 

 such as must have once existed hereabouts, should ever be 

 broken up by the protrusion of a mountain chain like the 

 Mount Wellington range, it is not unreasonable to suppose 

 that many fractures would take place at a distance from, 

 and parallel to, the eruptive mass. In addition to this 

 simple fracture of the beds, a lateral pressure may also have 

 led to a phenomenon somewhat analogous to what takes 

 place when the pieces of ice, in a large floe, commence to 

 pack one upon the other. 



The secondary fissures do not appear to have been filled 

 up with basalt contemporaneously with their formation. 

 The molten matter seems to have been run into them as 

 previously existing crevices. Still the filling up may have 

 followed closely, almost instantaneously, upon the shock 

 causing the fracture. 



Of the rocks of the district, the lowest is of an extremely 

 fossiliferous character, and is called mountain limestone by 

 the colonists. It is divided into two beds. Of these, the 

 lower is very calcareous, and contains a profusion of bivalves 

 (spirifera and producta) : the upper is more arenacious, and 

 is literally crowded with coral (fenestrella and stenopora^) 



It would seem as if the calcareous matter had granulated 

 from the upper to the lower beds. In the former, the re- 

 moval of the shells by infiltration, gives rise to a singularly 

 honeycombed appearance, wherever the rock is exposed in a 

 cliff. In the lower rock the matter of the original shells is 

 substituted by crystallised carbonate of lime. 



Above this formation is a dense compact stratum, locally 

 known as mud or clay stone. Mr. Selwyn estimates its 

 thickness at fully 400 feet. There are a few impressions of 

 shells found in it occasionally, but, generally, fossils are of 

 rare occurrence. 



Throughout the mass, at intervals, are a number of pieces 

 of angular quartz. The unworn condition of these would 

 suggest some method of transport other than mere power of 

 the water. Probably they were floated from a distance, en- 

 tangled in the roots of sea- weed. 



It was probably the prevalence of the turbid water giving 

 rise to this stratum which destroyed the corals, so numerous 

 m the upper beds of limestone immediately below. 



