Yol. 62.] GEOLOGY OF DT7NEDIN (NEW ZEALAND). 387 



All the exposed surface of the rocks has undergone much decom- 

 position, and rich soils have been formed. On these soils, a sufficient 

 rainfall and a mild climate have favoured the growth of a luxuriant 

 forest-vegetation, extending to the water's edge. On some of the 

 more exposed hill-tops, scrub and native grasses take the place of 

 the forest, and on the flat summit of Swampy Hill a luxuriant 

 .growth of moss so efficiently retains the abundant rainfall, that an 

 extensive peat-bog has been formed. During the last fifty years, 

 the growth of primaeval forest has been felled by the settlers, and 

 the hillsides are now clothed with a dense sward of grass ; but this 

 hides the rock-outcrops almost as effectually as the original forest, 

 and greatly increases the difficulty that must always be considerable 

 in attempting to solve the intricate relations between the many- 

 kinds of igneous rocks found here. 



II. Description op the Rock-Types, and their Geological 

 Occurrence. 



The rocks of the district belong to so many different types, and 

 the alternations and transitions from one type to another are so 

 frequent and abrupt, that much material will probably be discovered, 

 which will be of importance in testing the various theories that have 

 been advanced to account for the differences in structure and com- 

 position observed in igneous rocks in various regions. 



Many difficulties are encountered when one attempts to define the 

 range and extent of any lava-stream. In the first place, the thick 

 covering of vegetation and depth of soil prevent, over a great part 

 of the area, all possibility of obtaining specimens of the rock from 

 which the soil has been formed. The boulders that sometimes occur 

 on the surface are in many cases clearly not in situ, and examina- 

 tion of them is more likely to lead to erroneous than to correct 

 conclusions. 



Although microscopic methods show that many quite divergent 

 rock-types are present, the differences between them are often by 

 no means apparent in hand-specimens. Many of the rocks are so 

 fine-grained and of so uniform a black, that much field-practice 

 with these particular rocks is necessary before the different kinds 

 can be distinguished with any certainty. Many of the basalts are 

 so nearly identical in megascopic characters with some trachytoid 

 phonolites, andesites, nephelinitoid phonolites, trachydolerites, and 

 basanites, that field-observations and conclusions often prove to be 

 entirely incorrect. The late Prof. G. F. H. Ulrich, 1 when writing 

 of this district, said that the rock-varieties require 



' for their identification both chemical and microscopical examination nearlj 

 step by step/ 



On the other hand, there are many excellent sections exposed on 

 the lengthy sea-coast, while the small streams have everywhere cut 



1 Eep. Austral. Assoc. Adv. Sci. vol. iii (1891) p. 146. 



