64 NEPHELINE AND MELILITE ROCKS. 



We have noticed an extremely fine grained variety in' 

 which augite is dominant in the porphyritic form as well as 

 granular. This would appear to be an intermediate or 

 aberrant form tending towards the nepheline melilite 

 basalts. 



The families of nephelinite, nepheline-basalt, and meli- 

 lite-basalt are separated by Rosenbusch decisively from 

 ordinary basalts, with which, he says, they have no sort 

 of relation. He groups the three first-named families 

 genetically together, bound to each other by ties of 

 geological valency and association, and forming an integral 

 volcanic or efirusive formation, which (with the trachy- 

 dolerites, tephrites, leucite rocks, limburgites, and augi- 

 tites) l^elongs to theralitic magmas.* 



We may here add that we have not yet detected nephe- 

 line in any of the other Tasmanian basalts. The crystals 

 formerly attributed to nepheline in the Tertiary olivine- 

 basalts of Northern Tasmania have always seemed to, us 

 to be so invariably associated with longitudinal sections of 

 apatite as to make it probable that they were the hexagonal 

 transverse sections of the same mineral. A similar con- 

 fusion seems to have occurred with respect to the Tertiary 

 basalt of PhilliiJ Island, Bass Straits. In a letter recently 

 received from Prof. G. H. R. Ulrich, of Dunedin, he 

 informs us that the late Mr. Cosmo Newberry, not long 

 before his death, anah'sed the so-called nepheline of that 

 rock and found it to be apatite. One would, however,, 

 expect nepheline-basalts to be associated with nephelinite, 

 and it is highly probable that the Shannon district will 

 still be found to yield those lavas. 



Viewed from a mining point, these peculiar basaltic 

 rocks do not offer anything particularly encouraging. As 

 they are unique in Tasmania, there is little use in cona- 

 paring them with mineral-bearing rocks in other parts of 

 the island. The few localities in the world where such 

 rocks are known to occur are not noteworthy as mining 

 ones. The rocks are altogether incongruous with the 

 notion of tin ore occurring in them ; and though gold is 

 not intrinsically an impossible metal, distributed in 

 excessively small quantities as in some other eruptive 

 rocks, such as the Port Cygnet phonolitic trachytes for 

 instance, yet payable gold is, so far as we are aware^ 

 entirely unrecorded from this family of stone. 



* Ibid. p. 352. 



