81 



Microscopic study of the class, however, reveals the very frequent 

 existence of this mineral in small portions among the other 

 constituent minerals. In some cases its presence may be due to 

 deposition subsequent to the first formation of the rock, and such 

 belong to our next group. 



3. Quartz as an accidental constituent. 



By an accidental constituent is meant one that forms no 

 essential part of the rock, but has been introduced perhaps long 

 after that rock was formed or solidified. Its manner of occurrence 

 in these cases clearly shows that the quartz has been deposited 

 from solution — water containing silica infiltrating through the rock- 

 mass. It thus fills up cavities, and sometimes replaces other 

 minerals dissolved away. 



Among the Volcanic rocks of the District, quartz is very 

 common in this form. In beds of lava, and sometimes in those of 

 volcanic ash, vesicles or long almond-shaped cavities are generally 

 produced by the escape of vapour and gases from the molten or 

 heated matter, on its first eruption from the volcano. Subse- 

 quently when such lava beds have been covered up by great 

 thicknesses of overlying strata, the water which is for ever 

 percolating the crust of our earth, and contains very various 

 mineral substances dissolved in it, deposits these in the cavities 

 and vesicles, and amongst other minerals thus left to fill the vacant 

 spaces, quartz is very common, and may often be found filling the 

 same vesicle together with the minerals calcite (carbonate of 

 lime) and chlorite. When a large vesicle has been thus filled 

 with quartz, variousl)' coloured and under difTerent physical forms, 

 it sometimes happens that the surrounding rock is subsequently 

 broken up and destroyed, and then the hard kernels of quartz are 

 isolated in the form of agates. Such are the agates of U'allow 

 Crag, Keswick. 



// QUARTZ AS A MICROSCOPIC STVDY- 



Having enumerated the various forms and conditions under 

 which Quartz is found in the Lake District rocks, I wish to say 



