166 H. HICKS ON THE DIMETIAN AND 



only in containing a larger amount of these granular and chloritic 

 materials. Where the spherulites are but indistinct or not at all 

 developed the fluidal structure is still shown by the bands of the 

 dark granular mineral, the whole of the ground-mass in that case 

 being crystalline. Apatite needles also occur, but sparingly, and 

 are restricted to the vicinity of the crystals of felspar. 



Microlites, so characteristic of the younger rhyolites, do not appear 

 to be present ; otherwise the whole structure of the rock, in the 

 fluidal arrangement of the spherules &c, is so like that of a rhyolite 

 that it is difficult to believe that we have not before us one at least 

 of the many interesting varieties afforded by this group. One may 

 say with Zirkel, it is a rhyolite petrographically, but not geologically. 

 How long such a distinction can be maintained after the investiga- 

 tions of Allport and others upon other palaeozoic occurrences of this 

 rock, it is difficult to say. 



The cementing material of the agglomerate, as seen in the mi- 

 croscope, consists in the main of numerous broken crystals and 

 angular fragments of felspar and quartz enclosed in a fine-grained 

 ground-mass consisting largely of the dark opaque granular sub- 

 stance, with exceedingly minute grains of a depolarizing material 

 not to be recognized ; but, from its resemblance to the larger grains 

 of undoubted felspar, it is presumably felsitic. 



Another mass of the agglomerate consists of more or less sub- 

 angular fragments of a dark basic lava, slightly amygclaloidal, and 

 much altered ; the felspar crystals are exceedingly slender and 

 only occasionally afford evidence of their plagioclastic habit; the 

 ground-mass is black and opaque. The walls of the cavities are 

 lined with calcite ; and the interior is filled with a light-green clear 

 chlorite. The cementing material is only to be distinguished from 

 the enclosed fragments by its lighter colour, abundance of epidote, 

 and the presence of groups of crystals of angite. 



2. A compact dark-green rock, with a somewhat fissile aspect. 

 Yery little positive results are obtained by microscopic examination. 

 Scattered throughout a greenish-grey dust-like ground-mass without 

 action on polarized light, are innumerable minute grains and small 

 fragments of a tolerably clear crystalline mineral yielding bright 

 colours between crossed rTicols, which recall to mind the augite seen 

 in many extremely altered dolerites. 



There are no visible traces of a felspar, while the whole mass is 

 quite darkened with a granular nearly opaque substance quite un- 

 determinable, and is much veined with a pale yellowish-green 

 chloritic mineral. Can only be referred to the unsatisfactory group 

 of altered volcanic ashes. 



Discussion *. 



The President remarked that these papers showed what im- 

 portant results may be brought about by persevering industry ; it 

 was by this that Mr. Hicks, Prof. Harkness, and others had 

 gradually built up the system of the difficult and interesting country 



* This discussion relates to the three preceding papers. 



