162 PKOF. T. T. GEOOM ON THE IGNEOUS EOCKS [Feb. IQOI, 



larger than the felspar-laths, and are often numerous (PI. ^"^I, 

 fig. 6). 



The outer border (and in many cases the whole crystal) has in- 

 variably been resorbed, with the production of grains of iron-ores, 

 and probably of augite ; and this renders the accurate measurement 

 of angles a matter of impossibility. The faces of the prism meet at an 

 angle of about 124°. When the substance of the crystal is preserved 

 it is almost colourless, or of a very pale straw-tint, and accordingly 

 shows no perceptible pleochroism. The substance is very fibrous, 

 owing to the presence of two very close systems of cleavage parallel 

 to the prisms, and intersecting at an angle of 124°. So close is 

 the cleavage, indeed, that a relatively high magnification is required 

 to detect it in cross -sections. A. somewhat irregular jointing is seen 

 in needles cut parallel to the vertical axis. The fibres are often of 

 considerable length, and show strong refraction : they extinguish 

 at an angle of about 18°. The amphiboles are evidently closely 

 allied to, if not identical with, t rem elite in the asbestiform 

 condition. 



Original tremolite in igneous rook is described by Becke in some 

 of the Austrian serpentines.^ Prof. Rosenbusch,^ in speaking of the 

 hornblende of diorites, says : — 



' Relatively seldom the hornblende is of a very light green colour, and almost 

 devoid of pleochroism : it assumes then the character of " strahlstein," and 

 forms, not isolated crystals, but aggregates of needles.' 



Prof. Barrois, indeed, speaking of a ' diorite ' of this kind intrusive 

 in the Cambrian of Lago in Asturias, says that the long prismatic 

 and fibrous amphibole must be tremolite.^ In view of the usual 

 secondary nature of tremolite, it does not appear to be well-established 

 in such cases that the original species was tremolite. Colourless 

 hornblende is sometimes described as resulting from the bleach- 

 ing of ordinary hornbleude. Tremolite may arise from uralite, 

 or from green reedy hornblende,^ the latter itself being derived 

 from ordinary green hornblende or from brown hornblende (loc. 

 cit.). The Malvern tremolite might well be derived from a reedy 

 form of amphibole. This appears to be borne out by a com- 

 parison with the amphibole of the Warwickshire ' diorites.' In 

 these a colourless hornblende is described by Mr. Teall,^ and in 

 the rocks as seen in the Allport Collection, as well as in slides 

 kindly leut to me by Prof. Lapworth and Prof. Watts, all stages 

 may be found between crystals consisting of a brown reedy amphi- 

 bole and others composed of a reedy or asbestiform, colourless 

 tremolite. 



The substance of the amphibole in the Malvern rocks is never 



^ Tscherm. Min. & Petr. Mitth. vol. iv (1882) pp. 338 et seqg., 341 et seqq., & 

 348 et seqq. 



2 'Mikroskop. Physiogr. d. Massigen Gesteine' 3rd ed. (1896) p. 222. 



^ ' Recherches sur les Terrains Anciens des Asturies & de la Galice ' Lille, 

 1832, p. 124. 



* See Zirkel, ' Lehrbuch der Petrographie ' 2nd ed. vol. i (1893) p. 304. i 



5 ' British Petrography' 1888, p. 251. ■ 



I 



