Smeeth — On the Dolomite of Rowth. 273 



gravities, 2*825 and 2-835, and on analysis yielded the fol- 

 lowing : — 



Per cent. 



Calcium carbonate, . . . . 55-25 

 Magnesium carbonate, . . . 44*70 

 Ferric oxide, 0*53 



100-48 



The silica of the rock occurs in the form of minute but well- 

 formed prisms of quartz, with the usual pyramidal terminations. 

 It is chiefly confined to certain cavities in the rock to be mentioned 

 directly ; but some minute crystals occur disseminated through the 

 general mass : in the latter positions they are no doubt residual, 

 left behind from the mountain limestone ; in the former they must 

 have been deposited at the time of infilling of the cavities. 



The ferric oxide may have been originally deposited as ferrous 

 carbonate, which has since become altered by oxidation. It now 

 occurs as thin films, and in minute particles disseminated through 

 the rock. 



In common with most other magnesian limestones, the rock is 

 excavated by a great number of cavities. These are for the most 

 part irregular in shape and fortuitous in distribution. They are 

 lined, first of all with a film of iron-oxide, and next with well- 

 formed crystals of calcite, generally in the form of dog-tooth spar. 

 These then are the chief sources of the calcium carbonate which the 

 rock contains over and above that combined with the magnesium 

 carbonate in the form of dolomite. 



In addition to the preceding cavities there are others present 

 in the rock, of a somewhat different character : they do not occur 

 all through the rock, but only in certain regions : they appear to be 

 more abundant near the surface than deeper in the interior, and in 

 detached blocks than in the rock in situ, but this is probably due 

 to weathering, which may reveal them by oxidising any ferrous 

 salt they may contain. Their chief extension is along a series of 

 parallel planes, at right angles to which they are very narrow. 

 (Plate III., fig. 1.) 



Like the irregularly-shaped and distributed cavities, they are 

 lined with ferric oxide and calcite, and on any face of the rock at 

 right angles to their extension present themselves as a series of 



