72 GEOLOGY OF TASMANIA. 



columnar structure may be seen in the quarry near the 

 breakwater at Burnie. Dykes of this basalt traverse the 

 granite at Lottah, and at the summit of the Blue Tier. At 

 One Tree Point, Sandy Bay, a basalt is exposed which con- 

 tains the red-iron olivine fayalite, visible under a hand-lens, 

 as dark red spots (described by O. E. White and W. A. 

 Macleod). Basalt-glass, or tachylyte, occurs in the basalt 

 in several parts of the Island, e.g., Waratah, Richmond, 

 Bothwell, &c, No craters are known. 



The second variety of basalt is that forming the remark- 

 able bluffs at Circular Head and Table Cape. The late 

 Professor Ulrich at one time determined it to be nepheline- 

 bearing, but afterwards withdrew the reference to nepheline, 

 believing the mineral in question to be apatite. Apatite 

 is abundant in the rock, but recently microscopical examina- 

 tion has shown nepheline to be present also. The structure 

 is doleritic ; the mineral constituents are plagioclase -|- 

 augite -f- olivine + nepheline. 



The third type is melilite basalt, with typical nephelinite, 

 or nephelinite-dolerite, at the Shannon Tier, near Bothwell. 

 The geological horizon has not yet been determined, but the 

 age is believed to be Tertiary. 



4. Neogene. — The post-basaltic valley terraces can only 

 be separated from the earlier Tertiaries by position and 

 lithological characters. Some of the gravel drifts of the 

 Derwent, of the Longford plain, and in the neighbourhood 

 of Launceston, belong to this division. The lignite beds 

 of the Henty liiver contain leaves of Fagiis jonesii, (John- 

 ston) and Acacia rneiringii (Johnston), both closely re- 

 sembling existing species. 



The close of the Tertiary, or the beginning of the Quar- 

 temary, witnessed a glacier epoch in the western part of 

 the Island. The highlands round Barn Bluff, Mounts Tyn- 

 dal, Sedgwick, Jukes, Darwin, &c., and the western edge of 

 the great central plateau, abound with tarns, ice-scratched 

 stones, and moraines. Signs of ice-action have been traced 

 to sea-level on the West Coast, but the most abundant evi- 

 dence is to be found above the 2000-feet level. No proof 

 of glacier conditions m this period in the Eastern part of 

 the Island has been adduced yet. 



Tin-ore and gold-ore are the most important of the 

 mineral resources of the Tertiary system. These occur in 

 the alluvial gravels and leads of the period. The sands in 

 the Savage River, and other tributaries of the Pieman, have 

 been worked for osmiridium, and, at Mount Stronach, for 

 monazite. The zircon sand, near Table Cape, was also ex- 

 ploited a few years ago. Tertiary clays are used largely 



