Professor Duns on an undescribed variety of Amethyst. 351 



however, be well to carry the analysis farther by testing for 

 soda, magnesia, or manganese, having previously marked the 

 degree of intensity of the violet blue colour of the specimen. 

 But, apart from this, it will be seen from the pieces in the 

 lump and in section now exhibited, that they are made up as 

 follows : The base on which the crystals rest is a thin layer 

 of fine vesicular trap. Above this is a mass of highly 

 crystalline semi-transparent quartz, about an inch in thickness, 

 thickly packed but yet showing the planes of the crystals 

 less or more well marked, and, on the top of this a thin layer, 

 of granular-like amorphous quartz, out of which the definitely 

 crystallised amethyst proper seems to rise. This may or may 

 not be generally the order of the layers, but in the specimens 

 now before us it is well marked. The dirty red colouring 

 matter is confined to the faces of the hexagonal pyramids — the 

 characteristic crystalline forms of quartz — and is, for the most 

 part, deposited in pretty separate annuli, ring within ring. 

 In the specimens now under notice I have not seen any traces 

 of the " radiating spicules," referred to by Dr Dawson. Nor 

 are the rings on the same plane. When examined through a 

 good binocular they are seen to lie at different levels, a fact 

 which seems to w^arrant the inference that the highly 

 crystalline glaze, so to speak, in which they lie, consists of 

 different layers. The spots are not in all cases perfectly 

 circular, as may be seen by referring to the accompanying 

 plate. It would not be profitable to speculate on the 

 probable explanation of those molecular aggregations. I may, 

 however, ask the Society to look at the so-called morijholites 

 or clay concretions, and the dolomites from Cumberland, now 

 on the table, as illustrating, on a large scale indeed, in a 

 somewhat striking way, the close resemblance between them 

 in point of form and the spots figured on the plate. This 

 resemblance suggests a topic of great interest and of which 

 little has yet been made. I refer to the analogies between 

 the power of concretion and that of crystallisation. But I do 

 not wish to make more of this resemblance than to indicate 

 the fact. The crystalline matrix in which the spots occur 

 has, for want of a better term, been called a glaze. Is the 

 presence of this necessary in order to the iron oxide arranging 



