200 Proceedings of the Royal Physical Society. 



third. Dr Heddle has pointed out that the Tube of Escape 

 is always the last part to be filled. 



True agates, ^>.,those formed in closed cavities, are normally 

 slate-grey, lavender, or dove coloured, and are very rarely 

 coloured red or yellow through any ferruginous material 

 contemporaneously introduced. Vein-Agates, on the other 

 hand, which are formed in fissures and other cavities com- 

 municating more easily with surface-waters, often contain 

 iron as a colouring constituent. Where this is introduced 

 in only small quantities, and in the form of ferric oxide, it 

 occurs scattered throughout the chalcedony in the form of 

 minute spheroids. This gives rise to Carnelian, which is 

 blood-red and translucent Chalcedony. Where the iron, or 

 the other impurities, are introduced in larger quantities, so 

 that the Chalcedony is rendered opaque, Jasper results. No 

 true Opal appears yet to have been found in Vein- Agates in 

 Scotland. 



Much of the red coloration often seen in Scottish agates is 

 due to subsequent infiltration, as is clearly shown by many 

 specimens in the Scottish Agate Collection. The colouring 

 matter is usually ferric oxide, and it has probably been derived 

 from the Upper Old Eed. It affects the Chalcedony, and 

 not the Cachalong. 



For some reason which is not yet apparent, agates in 

 Britain, and probably elsewhere, are chiefly confined to ande- 

 sitic lavas, and they occur mostly in those of the (Caledonian) 

 Old Eed. It may be mentioned here that perfectly-formed 

 agates, evidently derived from the lower Old Eed lavas, 

 commonly occur in the conglomerates of the Upper Old Eed 

 Sandstone. The agates must, therefore, have been formed 

 at some time between the close of the earlier and the 

 commencement of the later period. 



Some of the changes that affect agates after their forma- 

 tion has been completed are of general interest, as bearing 

 upon other mineralogical questions. Amongst these is the 

 fact that the red staining just referred to is often bleached 

 out, wholly or in part, as a consequence of exposure at the 

 surface. As this change is regarded as not due to the action of 

 the humus acids, it may perhaps be referred to the prolonged 



