44 BR. J. W. EVANS ON A MONCHIQUITE PEOM [Feb. I9OI, 



In portions of the rock (including in some cases the greater part 

 of a microscope-section) both the circular areas and the interspaces 

 consist of a colourless isotropic substance of low refractive 

 index. It has no definite boundaries, and is quite continuous, 

 without any indication of break to mark the circumference of the 

 circles. These are defined only by the brown hornblende and the 

 augite that surround them, and occasional grains of nepheline which 

 play the same part as the coloured minerals. Between these the 

 clear isotropic material passes out, and fills the interstices of the 

 coloured minerals. The whole has a curious resemblance to a lake 

 nearly covered with floating vegetation, between which and in 

 occasional open spaces the clear water is visible. (See PL II, fig. 1.) 



In other places both interspaces and circular areas are made up of 

 anisotropic crystals. Kepheline — sometimes dark in all positions 

 between crossed nicols — is the chief constituent, but a variable 

 amount of orthoclase is also present. A felspar with very fine twin- 

 lamellation is occasionally seen, but not in sufficient numbers 

 for determination : it is not improbably anorthoclase (soda- 

 microcline). Microperthite can sometimes be recognized. 

 The felspar occurs most frequently in the interspaces, while in the 

 open circular areas there is nearly always an excess of nepheline. 



Elsewhere, while the interstices between the coloured minerals 

 contain orthoclase and nepheline, the larger circular spaces are 

 either entirely isotropic or partly so, and in part filled with nepheline 

 or orthoclase ^ ; but even in the former case the isotropic material 

 does not show definite crystal-outlines. In the case of one area 

 only where the anisotropic crystals were collected round the margin, 

 their inner (apparently idiomorphic) borders seemed to give a regular 

 polygonal shape to the isotropic space in the centre. 



This isotropic substance is in some places decomposed into small 

 colourless rod-like crystals, with parallel extinction and interference- 

 colours ranging from white to red and even blue of the first order. 

 The cleavage is parallel to the length.'^ The vibrations in this 

 direction have the greatest velocity of transmission ; and if the 

 crystal were uniaxial, and this were the direction of the axis, the 

 sign would be negative. This appeared to be confirmed by exami- 

 nation in convergent light, so far as the minute size of the crystals 

 would permit. The mineral is probably cancrinite,^ and in this 

 case it has every appearance of being an alteration-product. The 

 presence of cancrinite would exj)laiii the evolution of gas, pre- 

 sumably carbon-dioxide, when the powdered rock is treated with acid. 

 The nepheline, too, occasionally contains specks showing higher 

 interference- colours, which probably indicate incipient alteration to 

 cancrinite. 



^ la some cases idiomorphic nepheline is surrounded by the isotropic base. 



^ The terminations are not well-defined, and an indistinct transverse jointing, 

 which is not always exactly at right angles to the length, may sometimes be 

 observed. 



^ See [11] pp. 79, 80. The comparatively low interference-coloiu's are 

 explained by the iact that the crystals are usuallj^ too small to occupy the 

 whole thickness of the section. 



