﻿482 ME. J. A. THOMSON ON THE HOENBLENDIC [NoY. 1908, 



Germans. An analogous case of the derivation of an amphibolite 

 from an oliyinfels has been described by Dr. E. Dathe/ and 

 furnishes Prof. Eosenbusch with his argument for the frequent 

 derivation of hornblendefels from peridotites." 



At Glendalough a further change has taken place, whereby sphene 

 and pyrite separate out, and appear to deprive the hornblende of its 

 pigment, so that it passes from a green or brown hornblende into a 

 pale actinolite. 



Most of the hornblende existing in the amphibolite and a good 

 deal in the other types is clearly secondary, and from its idio- 

 morphic forms would be called ' recrystallized ' (fig. 3, p. 481). For 

 such cases Prof. P. Becke has proposed the term idioblastic.^ He 

 points out that, in the formation of the crystalline schists, the 

 relative idiomorphism is not to be considered as denoting the 

 order in which they have formed, since in the secondary formation 

 of minerals in a rock undergoing metamorphic reconstitution the 

 resulting minerals differ in their power of crystal-growth. He 

 specially instances the case of albite in the tonalite-gneiss of the 

 Zillerthal,"^ which in the struggle for space is allotriomorphic to- 

 wards the hornblende, but idiomorphic towards calcite and chlorite. 

 The use of Prof. Becke's term seems conducive to clearness of 

 description. 



(3) The Actinolite-Rock (fig. 4, p. 483). 



In this type the hornblende, instead of being massive, is split up 

 into numerous divergent fibres, often clustered together into tufts 

 and radiating groups. There are no signs of schistosity. The rocks 

 are in general light- coloured, and can often be seen to bear pale 

 minerals of the epidote-group in druses. In places the rock carries 

 a large proportion of chalcopyrite, and it is evident that it has been 

 aff'ected by the veins which traverse the adjacent schists. The 

 specific gravity is 3*04. 



In sections the rock is seen to consist almost entirely of 

 actinolitic hornblende, with a small amount of zoisite, sphene, and 

 sulphides. A single crystal of apatite was found. 



The amphiboles vary in diff'erent specimens from a colourless 

 tremolite to a pale actinolite. In types allied to the amphibolite 

 the actinolite-fibres are grouped in parallel clusters, and seem then 

 to represent original plates of hornblende. In the extreme types 

 the parallel clusters are subordinate to the radial and sheaf-like 

 bundles. Pine inclusions of sulphides are scattered throughout the 



^ [' Ueber Olivinfels von Habendorf bei Langenbielau in Schlesien '] Zeitschr. 

 Deutsch. Geol. Gesellscb. vol. xxxviii (1886) p. 913. 



2 ' Elemente der Gesteinslehre ' 2nd ed. (1901) p. 525. 



3 * TJeber Mineralbestand & Struktur der kristalliniscben Schiefer ' C.B. 

 IX^me Congres Geol. Internat. Vienna, 1903 (1904) p. 553. 



^ [' Vortrag iiber die krystallinen Schiefer der Alpen '] Tscherm. Min. & Petr. 

 Mittheil. n. s. vol. xxi (1902) p. 356. 



