xni THE MARIKO RANGE 187 



vesicular lapilli, up to half-an-inch in size, of a semi-vitreous basic 

 rock, the small steam-holes being either empty or filled with opal 

 or chalcedony. The matrix of the rock is made up of finer 

 fragments of a basic vacuolar glass, showing a few felspar micro- 

 liths, but often more or less palagonitised. Evidence of further 

 alteration is afforded by the small cracks and crevices filled with 

 chalcedony. 



Other altered tuff-rocks are exposed on the south slope. At an 

 elevation of 400-450 feet above the sea, and underlying the 

 agglomerates and breccia-tuffs, I found exposed in a stream-course 

 a hard dark rock looking like a compact andesite. Under the 

 microscope, however, it is shown to be an altered palagonite-tuff 

 composed in part of angular fragments of plagioclase and of 

 rhombic and monoclinic pyroxene, not exceeding -15 mm. in size, 

 and containing also similar-sized fragments of a basic hemicrystal- 

 line rock. The base is made up of palagonitic material and 

 contains a few " Globigerina " tests sometimes displaying calcite 

 in their interior. Fine cracks filled with chalcedonic silica testify to 

 a subsequent alteration of the deposit. At 1,500 feet occurs a hard 

 red altered palagonite-tuff, having a similar composition and being 

 altered in like fashion, but not displaying tests of foraminifera in the 

 slide. 



The foregoing remarks refer to the main undivided mass of the 

 range, that is, up to 2,000 feet. The highest peak of Mariko 

 probably represents in structure the other peaks rising to various 

 heights on either side of it. Here, at elevations between 2,000 feet 

 and the summit, a rubbly agglomerate prevails of a somewhat 

 different character from that occurring at lower levels. It is well 

 exposed in some cave-cliffs at a height of 2,500 feet and also in the 

 rocky face of the peak. The rock composing the blocks is a 

 dark-grey aphanitic augite-andesite (sp. gr. 2-65 ), referred to genus 

 20 of that sub-class and displaying prismatic pyroxene in the ground- 

 mass. Smoky residual glass exists usually in fair amount ; whilst 

 in the blocks of the cave-cliffs it is so abundant that the rock may 

 be termed semi-vitreous. In the locality just named the blocks are 

 scoriaceous, the steam-pores, which are drawn out to a length of 

 5 or 6 mm. and more, being partially or completely filled with 

 calcite and occasionally with opal. At times the steam cavities are 

 much larger. In one of my specimens there is an elongated cavity 

 5 cm. (2 inches) in length, which has a thin lining of chalcedony, 

 from the surface of which pyramidal crystals of calcite project into 

 the interior. (I found the same grey andesite exposed in situ lower 



