137 



de felspath." Such a definition is of course of little use in fixing the 

 meaning of the term as used at present. ZIRKEL (1) defines melaphyre as a 

 rock consisting essentially of oligoclase and augite. At the time this 

 definition was given there were no satisfactory determinations of the felspars, 

 and all that it can be taken to imply is that the melaphyres as a group were 

 more acid than the diabases and dolerites, which were described as composed of 

 labradorite and augite. ROSEN BVSCH defines melaphyre as a pre-tertiary 

 massive rock composed essentially of plagioclase, augite and olivine, together 

 with a certain amount of interstitial matter. The melaphyres according to 

 this definition are the precise equivalents of the basalts of the same author ; 

 the only difference being one of age. The presence of interstitial matter is 

 made the point of distinction between melaphyre and olivine-diabase. 



Mayina-lasalt. Name used by MOHL and B()RICKY ( ' 2) for a rock having 

 the chemical composition of basalt, but consisting only of augite, magnetite 

 and glass. Felspars absent, or if present occurring only in extremely small 

 quantity. 



Liiiiburf/ite. Name introduced by ROSENBUSCH (3) for a rock composed of 

 augite, magnetite and glass ; that is for a non-felspathic rock of basic 

 composition. 



P>/roxe>utc. Name proposed by DOELTER (4) for a rock composed of 

 augite, magnetite and glass. In this sense it is synonymous with BORICKY'S 

 magma-basalt. The term pyroxenite has been used in different senses by 

 different writers, and it has therefore been proposed that the term auyitite 

 should be substituted for pyrowiiitc (DOELTER). 



TacJtylt/te. Name introduced by BREITHAUPT in 1826 for what is now 

 known to be the glassy modification of the basic magma. 



Htjalomclanc. Name introduced by HAUSMANN in 1844 for a substance 

 supposed to be distinct from tachylyte. It is now admitted that there is no 

 valid distinction between hyalomelane and tachylyte, and the former name 

 is therefore dropped in favour of the latter. 



Sidvro/ticlfiH. Name introduced by Yon WALTERSHAUSEN in 1853 for a 

 tachylyte, from Iceland, which formed the anhydrous kernels of the same 

 author's palagonltc . 



Palngonifr. Name introduced by VonWALTERSHAusEN for a substance now 

 generally regarded as being formed from basalt-glass (tachylyte) by oxidation 

 and hvdration. 



CHARACTERS OF THE ROCK-FORMING MINERALS. 



Fekj)ar. Orthoclase is probably very rare as a constituent of the rocks of 

 this group. It is constantly mentioned as occurring by the earlier writers on 

 microscopic petrography ; but it must be remembered that the presence of 



(1) Lehrbuch der Petrographie. Vol.11, p. 39. 



(2) Studien an den Basalt-gesteinen Bohmens. Prag. 1874. 



(3) N.J., 1872. p. 35. 



(4) Die Vulkane der Capverden. Graz 1882. 



