282 Transactions. — Geology. 



2. StratigrapMcal Evidence. — The different varieties of rock pass gradually 

 one into the other, no line of division extending beyond a few yards having 

 been found, and no sequence of the different varieties can be traced, as can 

 always be done among stratified rocks. I think, therefore, that all varieties 

 of trachyte, porphyry, and bi-eccia must be considered as belonging to one 

 formation. Now this formation is spread over the greater part of the 

 peninsula, and attains a height of 2,600 feet above the sea; and a closer 

 examination of the district shows that the porphyries and other more 

 metamorphosed varieties of the trachyte are found only towards its lower 

 part, and do not extend up into the hills, as may be easily seen by examining 

 the mines at various levels in the Karaka and Tararu Creeks, where meta- 

 morphism has been most active. This cannot be due to decomposition of the 

 upper portions, for near the base of the formation the rocks are quite hard in 

 places where they have been exposed to the atmosphere for a long time, 

 ■while higher up long drives into the hills show that the rock there has never 

 been changed into a porphyry, for it contains no crystals nor ciystal cavities. 

 It is impossible to account for this fact on the supposition that the porphyries 

 ai*e older rocks tilted up, but it follows naturally from the supposition that the 

 whole mass is of volcanic formation, which was ejected in a heated state ; for 

 then the lower portions must have retained their heat longer than the upper, 

 and, as the felspathic nature of the rock renders it very liable to meta- 

 morphism, there is nothing extraordinary in finding the lower parts changed 

 into a porphyry. 



Those districts which are most metamorphosed are also most brecciated, 

 Buch as the Hape, Karaka, and Tararii Creeks, the beach north of the 

 Opitomoko, and the country about Tapu. This probably shows that the more 

 metamorphosed districts were nearer to the volcanic vents. The absence of 

 scorise is no argument against this view, for, as I have already said, the, 

 trachyte is more neai'ly allied to lava than to tuff, and it was certainly ejected 

 below the sea, while scorise can only be formed in the air. I have already 

 mentioned the great extent of country which this formation covers. This 

 alone proves either that it is nearly horizontal, or that it is thrown into 

 undulating curves. But the mines at the Thames have shown that it is crossed 

 in all directions by nearly vertical dykes, which have no particular direction of 

 dip in different localities, which would certainly be the case if the formation 

 was thrown into undulating curves. At Coromandel seams of coal have been 

 found at Sykes Gully, in the Kapanga townshij), and in the Hinau Creek, a 

 small tributary of the Matawai.* In the first case the coal dips N.N.W. 15°, 

 and in the second it is almost horizontal. In both places it is overlaid by 



* Both these beds of coal occur m the district marked as "greenstone tufa forma- 

 tion" in Dr. Hector's map of Coromandel. (Geo. Rep., 1870-71, p. 98.) 



