BY W. H. TWELVETREES AND W. F. PETTERD. 53 



small fragments of augite which are distributed in those 

 interstices. This is the intersertal structure of Rosenbusch. 

 Both ophitic and intersertal structures are met with in 

 basalts. Intersertal basalts are common all over the world : 

 ophitic ones have been described by Judd, from the 

 Western Isles of Scotland. Gabbros also are sometimes 

 ophitic. It would be interesting to know whether the two 

 modifications are characteristic of different geological 

 occurrences ; if so, one would expect to find the ophitic 

 structure prevailing in the larger masses of rock, and the 

 intersertal in narrow dykes. The most can be said of the 

 Tasmanian occurrences is that the intersertal structure is 

 confined to the close-grained varieties. 



Parts of this dolerite become converted into diabase 

 by the chloritisation of the augite. Thus a diabase on the 

 Blue Tier (Gould's Country) is a typical occurrence, the 

 whole of the augite, which is ophitic, being changed into 

 chlorite. Some of the occurrences of diabase in the island 

 may possibly be older, but evidence of their age is not yet 

 available. It may yet be. shown that some of these altered 

 dolerites are of Palaeozoic age ; for instance, there is a 

 dyke of diabase near the Hampshire Hills, and near the 

 Bridge on the Arthur River, Heazlewood, is a porphyritic 

 diabase, with ophitic chloritised augite, and a little light 

 brown biotite. The diabase occurring at the Blue Tier 

 may belong to the older series. On the Corinna Road, 8 

 miles from Waratah, is what appears to be a diabase with 

 intersertal structure. The felspars in it are smaller than 

 usual, and grains of augite occupy the interstices. Both 

 calcite and chlorite are present ; possibly this is a mela- 

 phyre. 



The European types with which the Tasmanian 

 Mesozoic dolerite may be compared, micro-structurally, 

 are the Hunne-diabase of Sweden, and the Kinne-diabase 

 from the Kinne-kulle of that country. The latter rock, as 

 exemplified by one of our slides, exactly corresponds with 

 our typical coarse-grained dolerite. In Sweden it covers 

 Silurian rocks in the form of a sheet. The Hunne-diabase 

 of the Hunneberg, in the same country, and in the same, 

 geological position, contains a little bronzite, biotite, quartz, 

 and hornblende, but in structure closely resembles some 

 •of our fine-grained varieties. 



Many people look at our bold escarpments and rugged 

 faces of " greenstone " and believe that some stupendous 

 eruption ejected the mass from below and poured it over 

 the land in an overwhelming flood. , In view of the 



