fkom tiik cross fkll inlikr. 517 



2. Basic L.vvah of Melmekhy. 



Tho porphyritic lava of Mclmerby presents in hand-specimens a 

 striking appearance, (xlassy-looking felspars, up to half an inch in 

 length, showing to the eye both Carlsbad and albite twinning, are 

 embedded in a dark compact ground. One is strongly reminded of 

 the well-known porphyritic lavas of Eycott^IIill, and closer exami- 

 nation leaves no doubt as to the identity of the rocks from the two 

 localities. The type is a rather unusual one, the composition being 

 decidedly more basic than that of normal andesites, while on the 

 other hand olivine is wanting, being in a sense represented by a 

 rhombic pyroxene with abundance of free iron-ore. Perhaps the 

 best name would be hypersthene-basalt. 



In specimens from Eycott, Mr. J. Hughes * found the silica-per- 

 centages 53-30, 52-60, and 51-10; Mr. T. Cooksey f found 53-40 

 and 52*73. The Mclmerby rock is so similar that an analysis is 

 scarcely necessary. A determination of its specific gravity gave 

 2-753, the figure for the Eycott rock being 2-754 (Cooksey, loc. 

 ciu). 



The porphyritic felspars are often grouped in such a fashion as to 

 interfere with one another's growth, while always presenting crystal- 

 faces to the surrounding matrix. They are on tho border-land 

 between labradorite and bytownite, being apparently a little more 

 basic than the type hafnefjordite, Ab^An^. In sections perpendi- 

 cular to the lamella) of albite-twinning the extinction-angles range 

 up to 37°, reckoned from the twin-line. This is for the greater 

 part of the crystal : the border gives a rather lower angle. As 

 regards habit, extinction-angles, zonary structure, inclusions, &c., 

 Mr. Teall's X accurate description of the Eycott felspars may stand 

 equally well for these. 



The chief feature of the groundmass is the plentiful occurrence of 

 a rhombic pyroxene, almost often to the exclusion of augite. This 

 is also the case in the Eycott rock, as was first pointed out by Prof. 

 Bonney {loo. cit.). Tho mineral is, however, invariably replaced b}' 

 bright-green fibrous bastite. The strong colour and pleochroism of 

 this substance, with the occurrence in it of abundant secondary 

 iron-ore, point to a ferriferous variety of pyroxene, hypersthene 

 rather than enstatite. The groundmass of the rock contains plenty 

 of magnetite. In tolerably fresh specimens [1250, 1251] this 

 shows the outlines of octahedral crystals, and is clearly original ; 

 but in more weathered examples the whole mass of the rock is 

 rendered almost opaque by the separation of secondary magnetite 

 with other decomposition-products. The felspars of the ground- 

 mass are probably not very dissimilar in nature to the larger 

 felspars ; they occur in small slender prisms, but always show 

 albite-lamellation, and do not sink to mere microliths. There 

 appears to have been some augite, in small granules now repre- 

 sented by calcite and a pale almost isotropic material ; and there 



* J. Clifton Ward, Monthly Microscop. Journ. (1877) vol. xvii. p. 246. 

 t T. G. Bonney, Geol. Mag. (1885) p. 80. 

 X ' Brit. Petrogr.' (1888) pp. 225-2l7. 



2o2 



