NEPHELINE AND MELILITE ROCKS FROM 

 SHANNON TIER. 



By W. H. Twblvetrbes, F.G.S., and W. F. Petterd, 



C.M.Z.S. 



We received i^ecently from the Mines Department some 

 specimens of rocks from the Shannon district, where they 

 had been looked upon as indicating the possible occurrence 

 •of tin and gold. Mr. George Allison, of Hunterston, kindly 

 supplemented these, and outlined for us their geological 

 -occurrence on that estate, and from his descriptions we 

 are able to indicate broadly the features of the locality. 

 The Shannon Tier forms there a high plateau of mesozoic 

 dolerite which rises a thousand feet above the Permo-Car- 

 boniferous country at its base. On the slope below the 

 Tier are small rounded or conical hills of a dark grey, 

 slightly bluish, basaltic rock : and at the iDase of or l)eneath 

 the flanks of these is a strange-looking coarse zeolitic rock 

 called locally " tourmaline-rock." This is as much as can 

 be stated at present respecting the geology of this rather 

 remote place. The presence of gold is said to have been 

 established in the tourmaline-rock, but an assay by the 

 Government i^nalyst did not confirm this. 



The locality gives us three varieties of eruptive rock, 

 viz., the Mesozoic dolerite, the so-called tourmaline-rock, 

 and the bluish basalt. We may here anticipate by diag- 

 nosing the pseudo-tourmaline rock as nephelinite, and the 

 basalt as melilite-basalt. The geological age of these rocks, 

 so far as can be hazarded without examination on the spot, 

 is probably Permo-Carboniferous for the nepheline and 

 m'elilite rocks. The dolerite is considered to belong to the 

 close of the Mesozoic era. 



Dolerite. — This varies in degrees of coarseness, but is 

 the typical ophitic dolerite which occupies the summits of 

 the Central Tiers, and of numerous mountains in every 

 part of the island. It is a holocrystalline plagioclase- 

 augite rock, structurally diabasic, and sometimes, where 

 the augite is chloritised, merging into diabase. The well- 

 formed prisms of labradorite felspar, sometimes long and 

 ;slender, sometimes stout and short, are cemented together 



