1905-1906.] 351 



forming the third layer, is the agate proper, which is composed 

 of chalcedony (of many colours), 'white agate,' cacholong, opal, 

 and crystalline quartz (usually amethyst), arranged in concentric 

 or level bands according to the nature of the siliceous material. 

 The intergrowth of the green earth with the chalcedony pro- 

 duces the well-known moss-agates ; and of iron-oxide with the 

 chalcedony produces the equally beautiful jaspers, with their 

 stellar, flammate. and pseudo-brecciate crystallisations. At 

 Hillend, on the Pentland Hills (near Edinburgh) an interesting- 

 variation of agate-building occurs in the cavities of an andesite. 

 The usual green-earth layer is present ; in this case of celadonite 

 and saponite, and as a second layer, in place of the zeolites, is 

 a deposit of dolomite, in a beautiful ramifying form, covered by 

 calcite, and within the latter is the third layer of chalcedony 

 (carnelian). A similar sequence of deposition is found in the 

 agates of Monzie (near Crieff, in Perthshire), described by Mr. 

 Kerr as 'amoeboid' agates. In this case the green earth is not 

 found at all. but its place is taken by a layer of iron oxide, 

 upon which the second layer of pearlspar has been deposited. 

 Within the pearlspar or dolomite is found the third deposit of 

 siliceous substance, constituting the agate of banded structure, 

 with various layers of chalcedony, quartz, etc. Mr. Kerr sug- 

 gests that these nodules have grown from the centre to the ex- 

 terior, and from within rather than from without ; but there is 

 little doubt, I think, that these growths have been formed ac- 

 cording to the sequence laid down in the above-mentioned law, 

 and that the 'hollow spherical depressions' on the exterior of 

 the Monzie type of agate, revealed by the weathering, or etching 

 away of the exterior layers of iron oxide and pearlspar, are 

 exactly analogous to the hollow pseudomorphous cavities on 

 the exterior of the Carnmoney chalcedony. 



