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commonly they are about the size of a walnut. When exposed to 

 the weather, nearly all these dolomites change through cream-colour 

 to saffron-yellow, or to dull ochreous tints, as a consequence of the 

 change of part of their iron into Limonite. Analyses of Dolomite 

 from our district shew a proportion of Iron carbonate ranging up 

 to ten per cent. 



At the Caldbeck Fell mining area ferriferous Dolomite is common, 

 along with the vein-stuff, perhaps as a result of the mingling of 

 magnesian solutions with others containing Lime carbonate re- 

 sulting from the decomposition of the volcanic- and other crystalline 

 rocks traversed by the veins, before the last of tlie New Red was 

 denuded from the plain now represented by the Fell tops there. 

 Dolomites with much the same general aspect, but weathering 

 with a beautiful rich golden-brown iridescent surface, are found at 

 Helton Mines, near Brough ; and specimens having the same 

 lustre and play of colours, from the West Cumberland area, were 

 presented a year or so since to the British Museum, by Miss S. H. 

 Bayles. 



Very pretty groups of madder-brown crystals were got from a 

 vein traversing the volcanic green-slates at the head of UUswater. 



The alteration of limestone into masses of a dolomitic character 

 has affected considerable thicknesses of rock in many instances. 

 In most of these the conversion, or replacement, has taken place 

 across the bedding of the rock ; thereby rendering it evident that 

 the dolomitic character is due to causes affecting the rock after its 

 deposition, and not, as some have supposed in the instances under 

 notice, to original differences of composition. The alteration in 

 some cases has proceeded so far as to convert bands, or dyke-like 

 masses of the rock into a crumbling mass of crystals, which have a 

 gritty feel to the touch, so as to bear no inconsiderable resemblance 

 to loose-grained sandstones. Instances are not wanting wherein 

 people whose education ought to have taught them better have 

 actually regarded these dolomitized limestones as veritable sand- 

 stones. The relative hardness of quartz grains to dolomite affords 

 a ready means of distinguishing between the two : where other 

 means are not available, the face of a geological hammer rubbed 



