158 A Physical Description 



disappeared in the course of time. This is only mentioned 

 to show that the reasons alleged by Mr. Gould are not deci- 

 sive of the question ; but, at the same time, he gives a much 

 more feasible explanation of the lake than a crater origin, 

 which most persons will be disposed to accept. According 

 to him the waters have been penned up in a natural valley 

 by a recent outflow of basalt. It is hardly to be supposed 

 that we should be able to trace the craters which have been 

 formed during the overflow of the greenstones. It may be 

 safely affirmed that the amount of basalt which has been 

 outpoured in recent times in New South Wales or Victoria, 

 or in South and South-east Australia, fully equals, if it does 

 not surpass, the greenstone deposits. The basalts are com- 

 paratively very recent, for they are but little, if at all, 

 altered, yet there does not remain a single crater in all 

 South-east Australia, and in the colony of Victoria they are 

 very few. It is only as we go westward to where the evi- 

 dence of volcanic action dies out that we find undoubted 

 ash craters with tufaceous deposits. 



In all probability the greenstones of Tasmania are ordinary 

 basaltic lavas, alternated by metamorphic action, or chemical 

 change, in which time and weathering were probably the 

 principal agents. After the researches of Mr. J. A. Phillips 

 on the " greenstones " of Cornwall, we cannot hesitate to 

 pronounce on those of the island. In Cornwall they are 

 proved to be lava, closely resembling those of modern date. 

 They are, in fact, dolerites, in which the augite has gradually 

 been transformed into hornblende and viridite*, while the 

 felspar merges into a granular mass. The titanic iron is 

 gradually replaced by a greyish-white product of alteration, 

 and a little epidote subsequently appears. The quartz, when 

 found in these, is a result of aggregation. No attempt has 

 been made to my knowledge to determine the character of 

 the Tasmanian greenstones. They are described thus by 

 Count Strzelecki (Loc. Git, p. 101) : — 



Diabase. — Brongniart ; Diorite, Haiiy. — The varieties of 

 this kind of rock are uniformly composed of felspar and 

 hornblende, in the state of grains or small crystals, in 



* Viridite. — This term refers to microscopic petrography, and is used to 

 express green or transparent substances visible in thin sections under the 

 microscope, forming scaly or fibrous aggregations, resulting from decompo- 

 sition of augite, hornblende, or olivine. The composition varies, but consists 

 chiefly of silicates of monoxyde of iron and magnesia. 



