84: PKOF. J. W. JTJDD ON TERTTAEX 



served as materials for these natural experiments, and have been 

 variously acted upon, so that there have grown up in the interior of 

 the cavities, crystals of the zeolites, chlorophseite, epidote, quartz, 

 calcite, and other minerals, with layers of chalcedony and other 

 crypto-crystalline materials. The removal of material from the 

 rock has often rendered it soft and friable, and when examined by 

 the aid of the microscope it is seen to have lost its characteristic 

 structure and mineral characters. I^ot unfrequently secondary 

 quartz or calcite is found to have been introduced into the substance 

 of the rock, giving it new cohesion, while the colour of the rock has 

 been changed from black to various shades of reddish brown through 

 the partial peroxidation and hydration of the magnetite. Some of 

 these altered rocks are in no way distinguishable from the amyg- 

 daloidal melaphyres and similar rocks which have usually been 

 regarded as being characteristic of the older geological periods. 



§ 4. Action of Atmospheric Agents ( Weathering). 



If the action of escaping steam and other gases on volcanic rocks 

 is only imperfectly exemplified in the district we are describing, 

 nowhere can we find better examples of the changes produced by 

 water percolating from the surface and carrying with it oxygen, 

 carbonic acid, and other substances in solution. It is not always 

 easy to separate the results of the two kinds of action from one 

 another or from the effects of the action of deep-seated solvents, 

 though their products are in some cases very strikingly contrasted. 



The felspars, as the result of weathering action, exhibit every 

 degree of kaolinization, owing to the breaking up of the silicates of 

 the alkalies and the alkaline earths, and the removal of their pro- 

 ducts in solution, this action being accompanied hy the hydration 

 and recrystallization of the silicate of alumina. Every stage of this 

 alteration can be traced, from a slight clouding of the transparent 

 felspars to their passage into white and opaque pseudomorphs. The 

 materials left behind in these pseudomorphs may recrystallize as 

 zoisite or other complex silicates, which have received the name of 

 "■ saussurite." In some cases the breaking up of the felspar mate- 

 rials is attended with the separation of considerable quantities of 

 silica, which crystallizes as secondary quartz ; such secondary quartz 

 is found in many of the altered gabbros of the district. 



The augites, both the non-ferriferous and the ferriferous varieties, 

 undergo similar very interesting series of change. Sometimes the 

 nearly colourless varieties acquire a brown or purplish tint on their 

 exterior portions, and this change of colour is seen extending 

 inwards in different crystals till the original colour is nearly lost. 

 It is evident from these facts that the colour of augite-crystals, as 

 seen by transmitted light, is a character upon which but little 

 dependence can be placed. The conversion of a very small propor- 

 tion of the iron-salt from the ferrous to the ferric state is probably 

 sufficient to produce considerable changes in tint. In some cases, 

 however, it is quite possible to discriminate between the original and 



