BY LEO A. COTTON 737 



preferential development, many junctions of granite and slate 

 being now obscured by basalt filling the ancient valleys between 

 these older formations. Several interesting cases were also 

 observed where the direction of the streams has been determined 

 by faults, and in some cases the master-joints have been 

 sufficiently well developed to produce a similar result. 



Chapter ii. — Geology. 



It will be convenient, here, to make reference to the plan of 

 the accompanying geological map, which is somewhat unusual. 

 The lode tin-deposits are associated with the older rocks of the 

 district, and have no genetic relations with the basalts, which 

 are much younger. Hence, it was considered best to construct a 

 geological map showing only such rocks as are concerned in the 

 origin of the ore-deposits. This plan necessarily makes the 

 geological map rather general, as the boundaries of the older 

 formations have to be approximated to where they are concealed 

 by the overlying basalt. Nevertheless the boundaries may be 

 accepted with conBdence as close approximations. Much of the 

 information, more especially with regard to the granite and slate 

 boundaries, has been very kindly furnished by officers of the 

 Department of Mines working in the same area. Where the 

 boundaries are shown in continuous lines, they have been accu- 

 rately determined, either by the officers of the Department of 

 Mines, or by myself. The broken lines represent that the 

 boundaries have been approximated to by numerous observations. 

 I have made a study of the two granite-types indicated on the 

 map, and in many places have traversed the contacts. Only 

 those ore-deposits which are specially mentioned in this paper, 

 are shown on the map. The work done on scores of smaller ore- 

 bodies has been condensed and generalised under the descriptions 

 of the difterent types of deposits. The dotted areas indicate 

 those in which tin has been found, and the density of the dotting 

 is intended to convey an idea of the concentration of the 

 cassiterite. 



Two sections accompany the geological map. These, it should 

 be remembered, are drawn in harmony with the plan of the map, 

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