324 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



prisms from Faroe ; they are always perfectly transparent, 

 and may sometimes be detached in a complete state, with 

 both their terminal summits. 



In examining this variety of Apophyllite, of which I was fa- 

 voured with a very fine specimen from the excellent cabinet 

 of Dr Macdonald, I was enabled, by the perfection of the 

 crystals, to study their structure, through the natural planes, and 

 at right angles to their axes. The phenomena which this inves- 

 tigation presented to me, were of a very singular and unex- 

 pected nature. In symmetry of form, and splendour of colour- 

 ing, they far surpassed any of the optical arrangements that I 

 had seen, while they developed a singular complexity of struc- 

 ture, and indicated the existence of new laws of mineral orga- 

 nisation. 



When a complete crystal of this variety of Apophyllite is ex- 

 posed to polarised light, with its axis inclined 45" to the plane 

 of primitive polarisation, and is subsequently examined with 

 an analysing prism, it exhibits, through both its pair of pa- 

 rallel planes, the appearance shewn in Plate XXI. Fig. 1. In 

 turning the crystal round the polarised ray, all the tints vanish, 

 reappear, and reach their maximum at the same time, so that 

 they are not the result of any hemitropism, but arise wholly 

 from a symmetrical combination of elementary crystals, possess- 

 ing different primitive forms, and different refractive and polar- 

 ising powers. The difference in the polarising powejs is well 

 shewn by the variation of tint; and the difference of refractive 

 power may be observed with equal distinctness, by examining 

 the crystal with the microscope, under favourable circumstances 

 of illumination, when the outlines of the symmetrical forms, 

 shewn in Plate XXI. Fig. 1. will be clearly visible. 



In examinincf the splendid arrangement of tints exhibited in 

 the figure, the perfect symmetry which appears in all its parts 



is 



