﻿Yol. 64.] EOCKS OF GLENDALOUGH AND GREYSTONES. 479 



that the brown variety is original, and the green an alteration- 

 product of the former. 



A peculiar structure resembling coarse schillerization is common 

 in the brown hornblende. It consists of blotches or fine rows of 

 iron-ores crossing the cleavage-traces obliquely. Similar structures 

 are found in the hornblende-peridotites of Molenick (East Cornwall) 

 and of Penarfynydd (Anglesey).^ It seems certainly in the former 

 case to result from the magmatic resorption of the augite and its 

 replacement by the hornblende, and may be ascribed to the same 

 process here. 



Ohio rite-minerals occur in two habits. In the first their form 

 in large plates with ragged edges suggests that of a mica, and an 

 occasional increase of the birefringence strengthens the probability 

 that the chlorite is replacing this mineral. The other is associated 

 with olivine, which it penetrates in small laths, and represents 

 with tremolite an alteration of that mineral. This chlorite is 

 less pleochroic, but more birefringent than the other. 



Mr. H. E. Clarke, B.A., of Jesus College, Oxford, has kindly 

 analysed this rock with the following result : — 



I. 11. III. Mean. 



SiO^ 43-39 43-32 43-52 43-41 



AI2O3 9-09 ... 9-12 9-10 



Fe^Og 9-31 ... 9-46 9-38 



FeO 7-25 7-26 724 7*25 



MgO, 16-80 ... 16-43 16-62 



OaO 9-22 9-30 9-55 9-36 



Na^O 0-04 ... ... 0-04 



H^O-h 3-45 ... ... 3-45 



H2O- 0-14 ... ... 0-14 



S 0-56 ... ... 0-56 



CO2 0-09 ... ... 0-09 



Totals ... 99-34 99-40 



Traces of oxides of titanium, chromium, manganese, and nickel, also of 

 potash and fluorine, were noted. Phosphorus-pentoxide, zirconia, and chlorine 

 were looked for, and found to be absent. 



Between this type of rock, a somewhat sheared but still 

 recognizable hornblende-peridotite, and the reconstituted amphibolite 

 to be described later, there is every stage, the most interesting being 

 an augite-amphibolite still retaining the evidence of poecilitic 

 structure (fig. 2, p. 480). It contains large plates of brown horn- 

 blende passing locally into the green, and crowded with small 

 rounded patches consisting of tremolite grown in optical continuity 

 with the enclosing plate. That these patches mostly represent 

 olivine may be seen by tracing all the intermediate stages. Often 

 confused rounded aggregates of chlorite, spotted with magnetite 

 and pyrite, show a less complete alteration of the olivine, of which 

 unaltered kernels may be observed. These aggregates are usually 



^ J. J. H. Teall, ' British Petrography ' 1888, p. 97. 



