On the Genesis of Some Scottish Minerals. 201 



action of weak solutions of nitric acid derived from the soils. 

 Another change is molecular, and arises from the tendency 

 of opaline silica to part with its combined water, and thus to 

 pass into the crypto-crystalline, or even into the crystalline, 

 condition. There is usually some loss of volume with this 

 change, and in a few instances the layer affected develops a 

 kind of jointed structure, through the formation of divisional 

 planes perpendicular to the bounding surfaces of the layer in 

 question. 



After agates have been affected by weatheriug at no great 

 depth below the surface, percolating waters sometimes carry 

 in solutions of various substances into any crack which may 

 have been formed within the agates. Amongst these solu- 

 tions, those of manganese are by no means uncommon in 

 certain localities. From these there is often deposited the 

 hydrous oxide, which, in passing into the solid condition, 

 usually does so in dendritic and moss-like forms. These are 

 usually black or brown in colour. Agates containing these 

 dendrites are called Mochas. The true Moss Agate is a 

 contemporaneous structure, is normally green in colour, and 

 is due to Celadonite enveloped in Clear Chalcedony, as 

 already mentioned above. 



Under the influence of contact metamorphism, agates pass 

 into crystalline Quartz, and their Celadonite into Epidote. 

 This is well seen in some agates from near Tillicoultry, as 

 well as at Glencoe. The Withamite of the Old Eed lavas 

 of Glencoe is clearly due to this cause. 



The various stages in the history of an agate are full of 

 interest, not only for the student of Mineralogy, but also to 

 the geologist, as many of the processes which have operated 

 on a small scale in the case of an agate exemplify what takes 

 place on a scale of much greater magnitude in connection 

 with rock-masses of all dimensions. 



The formation of siliceous concretions, such as the nodules 

 of flint and chert in limestones, has been brought about by 

 processes which in all essential particulars agree with those 

 concerned in the formation of agates. In the case of the 

 flint and chert, however, the materials have often replaced an 



