179 



is in the form of close-set fibrous masses of horny-looking, tea-green 

 crystals, which extend across the veins from one side to the other. 

 In many cases they appear to be of very little later date than the 

 bed wherein they occur ; and they probably represent the date of 

 the contraction of the rock consequent upon its passing into the 

 solid state. 



Olivine, the Di-magnesian (ferrous, etc.) Silicate, a mineral that 

 forms an important constituent of many modern rocks of volcanic 

 origin, is rare, or, at the most, represented only by pseudomorphs 

 in most of the Older Palaeozoic rocks of the Lake District. In 

 some eruptive rocks of Post-Carboniferous age in North Cumber- 

 land it has lately been detected in small quantities. 



Amongst the Hydrated Monosilicates is Serpentine, which 

 as a separate mineral, can hardly be claimed as a local species ; 

 though compounds of much the same general nature occur as 

 decomposition-products of parts of eruptive rocks throughout 

 nearly the whole of the Older Palaeozoic rocks of the district. 



Talc, the Hydrous Magnesian monosilicate, is stated by Bryce 

 M. Wright {op. cit. p. Ixxx.) to occur "associated with haematite, 

 near Egremont." As a decomposition-product it occurs under the 

 same conditions as the mineral last-enumerated. 



Hemimorphite, Smithsonite, the Hydrous Zinc monosilicate 

 has been obtained from two localties in the district. At one, 

 Alston, it occurs in the form of light-grey to white, encrusting 

 groups of minute acicular crystals, coating blende and other 

 minerals, in connection with the fault-breccia of mineral veins. 

 The other, and better-known locality for the mineral, is Rowtin 

 Gill, in the Caldbeck Fells. Here it occurs in the form of en- 

 crusting sheets composed of small acicular crystals set side by side. 

 These sheets of crystals are disposed layer over layer, to a thickness 

 of sometimes as much as an inch or more. The outer surface is 

 usually smooth ; but occasionally the outer ends of the separate 

 crystals project so far as to form a surface more or less rough. 

 Masses have been found extending over a surface of nearly a 

 square foot in area. Here too, as at Alston, this silicate is inti- 

 mately associated with fault-breccia in the mineral veins of the 



