206 E. HILL AND T. ft. BONNEY ON TEE 



across the quarry ; that it is indeed considerable is proved by micro- 

 scopic examination. The groimd-mass of this rock resembles gene- 

 rally, in its minuter structure, both those last described, but is more 

 streaky, showing the rounded granules both more rarely and less 

 distinctly. "With crossed Nicols the difference is more marked- — a 

 considerable portion of the ground-mass remaining dark, but being 

 crowded by minute, ill-defined, clear spots, and many small felspar 

 crystals, resembling fragments, many of these being long and narrow, 

 like clippings of thread. Thus, at first sight, the ground-mass much 

 resembles that of an igneous rock ; but closer inspection shows that 

 the granules vary rather more in form and size, and the felspar 

 microliths are more ragged at the edge than is usual in lavas. The 

 same minerals as in the above-named rocks occur porphyritically ; 

 but the felspar crystals are rather better preserved, though some are 

 almost replaced by epidote, which shows remarkably beautiful 

 colours ; the edges of others have a fractured aspect. The quartz 

 grains are reduced in size and still more in number. 



Barclon Hill, Compact Rode of Upper Pit (p. 781). — This rock 

 is of a greenish or greenish-grey colour, indistinctly mottled, show- 

 ing small glancing felspar crystals, and resembling a rather compact 

 felsite. Under the microscope it has a general similarity to the 

 rock last described, except that the felspar crystals (apparently of 

 both kinds) are smaller and more altered, and the abundance of 

 minute epidote, intercrystallized with earthy granules, rather ob- 

 scures its structure. 



Birchwood Plantation (p. 771). — This rock has considerable resem- 

 blance to the last described, and the microscope shows it also to 

 have undergone great alteration. There is the usual clear glassy 

 base, with dusky granulations, which here and there seems to indi- 

 cate the former presence of fragments by their aggregation in irre- 

 gular lines. Although on crossing the Mcols the slide, as a whole, 

 shows an imperfectly crystalline structure, which might belong to 

 some lavas, yet here and there, in the apparent fragments, there is 

 a marked difference, such as long acicular crystals, instead of the 

 more irregular forms. Larger felspar crystals are present, apparently 

 of both kinds (much altered in the usual way), with a little fibrous 

 hornblende (?) and still less quartz. On the whole it is more pro- 

 bable that this is only an altered rock ; and this is quite in accord- 

 ance with its appearance in the field. All the rocks of this group 

 so closely simulate the structure of an igneous rock, and have been 

 so much altered, that it is very difficult to recognize their true cha- 

 racter. We believe, however, that careful study, both in the field 

 and with the microscope, will prove they are of clastic origin, 

 though probably volcanic materials have entered largely into their 

 composition. 



(5) Fragments included in Breccias. — In the north-western portion 

 of the Eorest (coloured on the Survey Map as a felspathic porphyry 

 passing into altered Cambrian strata) breccias are numerous, as 

 described in our former paper (pp. 772-779). Two kinds of rocks are 

 there stated to be abundant, both resembling felstones — one a com- 



