Vol. 56.] IN FEEDERICK HENRY BAY (TASMANIA). 335 



An examination of the section, however, reveals very different 

 conditions. The sedimentary rocks to the left have been so 

 indurated and altered, that the planes of stratification are in many 

 places traceable only on the weathered face. From the same 

 cause the rock, at first sight, appears to be barren of fossils ; 

 but I found sufficient traces of Fenestella, Spirifer, Productus, etc. 

 to identify it with the limestones interstratified with shaly bands, 

 which constitute the lower members of the Permo-Carboniferous 

 Series in South-eastern Tasmania. The shale has been converted 

 into chert, and the limestone in some places into an intensely 

 hard whitish marble. The direction of their dip is about west- 

 south-west, and the face of the section is nearly in line with 

 their strike. They occupy the whole of the rocky point to the east 

 and south of the section, the mudstones and sandstones of the upper 

 portion of the marine series showing themselves to the south and 

 west at no great distance, with some indications of an intervening 

 fault throwing up the lower portion of the series. 



The dark rock to the right, in the lower part of the section (see 

 PI. XXII), is the ordinary diabase of Eastern Tasmania, showing 

 the finely crystalline, granular structure, which is noticeable in this 

 rock wherever it is found in contact with the original cooling- 

 surface. Its main constituents are plagioclase-felspar and augite, 

 and a microscopic examination would probably disclose the ophitic 

 structure, which has been shown by a competent lccal authority, 

 Mr. W. H. Twelvetrees, to be an invariable characteristic of the 

 Tasmanian diabase. The rock is rudely columnar where exposed 

 on the coast, and it extends for a few hundred yards to the north 

 and west, being succeeded by Permo-Carboniferous mudstones 

 apparently undisturbed, but extensively denuded. The junction is 

 hidden from view by the mud-flats and sand-dunes of Ralph's Bay 

 Neck. 



The diabase, with a more coarsely crystalline structure, appears 

 again at many points within a radius of a few miles in the 

 massive form described in the first part of this paper. Of its 

 intrusive character at the point described there can now be no 

 doubt, and the evidence afforded by the section seems to show that 

 this is not a case of an ordinary lateral thrust. It suggests rather 

 that the whole mass of the altered rocks has been bodily lifted 

 from its original position by the intrusive sheet, and that, to match 

 the peculiar fracture of the bedding-planes exhibited by the 

 section, there must be, far down below the present sea-level, a 

 corresponding series of steps, or ' benches,' in the undisturbed 

 formation, from which the portion now visible on the surface has 

 been torn away. 



The other section mentioned by Jukes is on the face of a lofty 

 precipitous cliff to the east of the entrance of Port Arthur. The 

 place is reported to be inaccessible for any purpose of close exami- 

 nation ; but on the other side of the estuary I have noted many 

 interesting sections in which the diabase occurs as an unmistakable 

 sill,' with alter ed sandstone immediately overlying it. 



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