BY LEO A. COTTON. 749 



the denudation of the earlier lavas. This series has been shown 

 to attain a thickness of about 200 feet. 



This period of quiescence was followed by another outpouring 

 of lava which buried swamps and marshes, and so preserved 

 abundant plant-remains. 



Both periods of volcanic activity gave rise to numerous separate 

 flows of lava. No less than eight flows can be distinguished in 

 the neighbourhood of Elsmore. 



Recent Alluvials. — The alluvial deposits lie naturally along the 

 present watercourses. The extrusion of the basalts filled the 

 main valleys developed in the '^' Canon Cycle," and as the streams 

 have not yet had time to mature, there are, in consequence, no 

 large alluvial deposits. Dredging is vigorously carried on along 

 the beds of the creeks, but the payable ground is seldom 100 

 yards in width. The depth of alluvial is rarely more than 40 

 feet, while the tin-bearing portion is seldom as many inches in 

 thickness. Nevertheless, it is from these recent alluvials that 

 the great bulk of the New England tin has been won. 



Rocks of Undetermined Age. 



Quartz-Porphyry. — Flanking the eastern margin of the tin- 

 bearing district, there has been found, in several places, a typical 

 quartz-porphyry. The junction of the rock with the granites and 

 slates was unfortunately covered by basalt where v'er observed ; 

 hence its age cannot be determined from its relation to these 

 rocks. The trend of this belt is, as may be seen from the map, 

 nearly north and south. 



FeUpar-Forphyry. — The quartz-porphyry just mentioned, is- 

 bordered partly on its eastern side by a related, but quite distinct, 

 type of rock. This rock is a felspar-porphyry, quartz being 

 almost entirely absent The phenocrysts are plagioclases of 

 about 2 mm. in diameter. It is best developed in the neighbour- 

 hood of Wandsworth. 



Quartz-Orthoclase-Forphyry. — Still further to the east, a very 

 striking quartz-orthoclase-porphyry was observed. The orthoclase- 

 crystals are of a pink colour, and are fi'om 2 to 3 mm. in length. 



