332 Geology of 



the general term. " basalt." Of secondary minerals, we find, either lining 

 or filling cavities, spha?rosiderite, carbonate of lime, (in the form o£ 

 calcareous spar and aragonite, of which the latter is the younger), 

 chalcedony, hyalite, opal, jasper, natrolite, mesotype, iron pyrites, 

 and several others. 



The internal structure of the lava-streams is generallv massive, but 

 being divided by more or less numerous joints into polvhedric blocks 

 some of them have a tendency to spheroidal structure, each polyhedric 

 piece exfoliating with concentric layers with a hard kernel in the 

 centre ; such a structure is principally observable when the rock is 

 partially decomposed. 



Dr. Hector, at my request, had a number of specimens obtained by 

 me during the tunnel survey, analysed at the Colonial laboratory at 

 Wellington, the main results of which, embodied in three tables, will 

 be found attached to this chapter. I have added to Dr. Hector's 

 nomenclature, based without doubt upon the amount of silica con- 

 tained in each specimen, the terms by which I designate them in 

 accordance with the occurrence, character, and nature of each. I 

 have also added the analysis of a remarkable trachyte, formino- a 

 lava -stream of considerable size, and having an average thickness of 

 eighty feet, which is interstratified between two others of a basic 

 character. This peculiar stream occurs between Lyttelton and the 

 pass to Sumner. It is the only trachyte lava known to me as having 

 flowed from any of the different centres of eruption of Banks' 

 Peninsula, all the other acidic rocks, as I shall show in the sequel, 

 having been ejected into fissures of more recent date. This lava- 

 stream consists of a white vesicular trachyte rich in quartz, res emblino- 

 closely some of the domites of the Auvergne, from which, however, it 

 is distinguished by its larger amount of silica, although it approaches 

 it again in its considerable percentage of potash. A vertical dyke, 

 about eight feet thick, of a peculiar flaky, silky trachyte passes through 

 this lava-stream, narrowing, however, in its upper portion. Although 

 this acidic lava is rather soft and friable in small pieces, it has never- 

 theless resisted the disintegrating agencies at work far better than 

 the hard basaltic lavas and agglomerates in its neighbourhood. The 

 analyses of Xos. 10 and 191a, the specimens for which were taken 

 from two small basaltic lava-streams, agree very well with the 

 analyses of basaltic rocks from extinct European volcanoes, made 

 by Eammelsberg, Engelbach, Streng, and others. The rock 'No. 180 



