BY LEO A. COTTON. 499 



The basalts are the youngest foimations, and, like the clay- 

 stones, do not occur closer than a mile or thereabouts to the 

 mine. Even at this distance there is no great development of 

 basalt, the occurrence being of the nature of cappings to Tertiary 

 leads. The basalts themselves have been clearly demonstrated 

 to belong to the Tertiary period. 



It is with the granites that the ore-deposits are concerned; and 

 of these granites there are two main types. The most important 

 in this discussion is the " acid granite," previously mentioned; 

 and the other has been designated, by me, the Tingha Granite. 

 The latter is certainly the older rock, and has been intruded by 

 the " acid granite." It is a hornblende-biotite granite, with large 

 porphyritic crystals of plagioclase felspar. The " acid granite " 

 consists almost entirely of quartz and felspar in its type develop- 

 ment. Various phases of it contain small proportions of biotite, 

 and tourmaline is occasionally present. Micrographic inter- 

 growth of quartz and felspar is a very characteristic feature. 



A small part of the generalised map of the district (see text- 

 fig.3) given in a former paper'^' is reproduced to show the approx- 

 imate relationships of the geological units in the vicinity of the 

 mine. The boundaries here have not been traversed, and are 

 only crude approximations; yet they are sufficiently accurate for 

 the purposes of this discussion. 



Ore-Deposits. 



It will be seen, by reference to the map, that the ore-deposit 

 at Borah Creek occurs in a tongue of " acid granite," at no great 

 distance from the contact formed by its intrusion into the Tingha 

 Granite; indeed the south-eastern portion of the ore-deposit 

 crosses the j a action of the granites. The lode, though readily 

 traceable on the surface for over two miles, has, for such a per- 

 manent ore-body, quite an insignificant outcrop. Towards its 

 north-western limit the main lode is joined by another, known 



* Cotton, Leo A., "The Tin-Deposits of New England, N.S.W. Part i. 

 The Elsmore-Tingha District." Pioc. Linn. Soc. N.S.Wales, 1909, p.733- 



