The mineral tridymite appears to have been first recognised by Rutley in these fine grained rocks. He remarks of them in general: “Reheating of already solidified lavas has been, as we have frequently had occasion to remark, a by no means uncommon feature in the history of these rocks.” He also speaks of “a globulitic devitrification” in connection with this. (Rutley, 1899, p. 465.) Sollas disposed of this idea of refusion so far as certain features of the more coarsely spherulitic rocks are concerned; but substituted for it the explanation that a certain peculiar decomposition had taken place. (Sollas, 1905, p. 121, vol. 1.) While Rutley seemed to regard the fine grained rocks as lava flows, with two exceptions which he called “tufaceous rhyolites.” Sollas treated them all as lavas; and this opinion was apparently held by Morgan, Henderson, and Bartrum also. (Morgan, 1912, 1913, 1924.) In the last of these bulletins Morgan gave a peculiar explanation of an unusual feature of these “tridymite rhyolites.” All observers had noticed the occurrence of triclinic felspars almost to the exclusion of monoclinic forms, and Morgan had previously recorded the presence of carbon in some of these types. In the publication quoted he refers to the crystals as “clearly showered on the lava, or surface derived, like the fragments of carbonised wood previously mentioned.” (Morgan, 1911, p. 67.) It is merely remarked at the moment that the common occurrence of plagioclase, and the far less common occurrence of quartz, suggests that the magma from which the ignimbrites were derived was of the nature of a highly acid dacite. This type has its most typical development at Arapuni. It is also found at Ngutuwera and widely near Waihi. In all of the samples of this fine-grained type or arapunite that were examined by Rutley, Sollas, Morgan, Henderson, and Bartrum, it seems that fragments of glass were practically absent. In other parts of the district, however, it is found that glass shreds and curved glass fragments of minute size compose the whole of the fine grained matrix of the rock; though in hand specimens the rocks can hardly be distinguished from the typical “tridymite rhyolites” of Waihi. Though actual shreds and fragments of glass are not seen in these “tridymite rhyolites,” there can be no question that the ill-defined axiolites, which constitute their main material, were originally glass shreds; though in them incipient crystallisation was developed at the time of their deposition. These minute particles of glass at Arapuni always have their greatest dimensions in the horizontal plane. They are pale brown in colour, and in section are seen to bend round the corners of the felspar crystals, thus giving the typical appearance of the so-called flow structure. (Plate 63, fig. 1.) Fayalite occurs in types of these acid rocks at Ohena Island, Mercury Bay, at Pohaturoa, Atiamuri, and is perhaps altered to magnetite in many other localities.