182 ME. H. H. AENOLD-BEMEOSE ON A QUAETZ-EOCE [May 1898, 



The irregularly-shaped bosses and the veins of quartz-rock, and 

 the juxtaposition with them of the quartzose limestone, are results 

 ■which might be expected if the silica was derived from thermal 

 waters. This theory would also explain both the origin of the 

 silica and the removal of the limestone w^hich it has replaced. 



EXPLANATION OF PLATES XI & XII. 



Plate XI. 



Map of the district north-east of Bonsall, on the scale of 12^ inches = 1 mile, 

 showing the distribution of the quartz-rock and the quartzose limestone. 



Plate XII. 



(The figures are photograjDhed by the author from the microscope, the first 

 five in polarized light and between crossed nicols, the sixth in ordinary light.) 



Fig. 1. Quartz-crystals in fluor. Pindale (g). Traces of cleavage-planes of 

 the latter mineral are seen in the dark portion of the figure. 

 Thin slice No. 586. (See p. 176.) 



2. Quartz-rock. Top Lift (a). The quartz-grains are elongated in the 



direction of tlie principal axis, and extinguish with their length. 

 They do not possess crystalline outline, but closely interlock. 

 Thin slice No. 420. (See p. 171.) 



3. Quartz-rock, Pindale (g). Similar to fig. 2, but of a more finely- 



grained structure. Thin slice No. 598. (See p. 176.) 



4. Quartzose limestone. Chert-quarry, Masson (b). Part of a tortuous 



vein of quartz in a base of cloudy calcite. Thin slice No, 534, 

 (See p. 172.) 

 6. Quartzose limestone. North of Chert- quarry (c). Quartz-cr3'stals 

 containing calcite-zones. A large quartz -grain is seen with zones 

 marking successive stages of growth. Thin slice No. 427. (See 



P- 1'2.) 

 0. Quartzose limestone, Pindale (ff). A foraminiferal limestone showing 

 an organism pierced by a quartz-crystal. Thin slice No. 589. 

 (See p. 177.) 



Discussion-. 



Prof. Bonnet said that he thought the Author had quite made 

 out his case. The chert in the Carboniferous Limestone was 

 perhaps a stage in the same direction ; the Upper Greensand 

 at Ventnor was converted in a similar way into chert, and the 

 Portland oolite was locally converted into chert. It would be 

 interesting to know whether the process now described was by trans- 

 ference of silica from organisms, or was from thermal springs, as 

 was probably the case in the Stotfield sandstone near Elgin. 



Prof. Watts said that he had observed several cases of the 

 growth of quartz-crystals in limestone, though the process had 

 never gone quite so far as the extreme cases mentioned by the 

 Author. Instances from the South and Centre of Ireland and from 

 the Purbeck Limestone indicated some widespread cause, rather than 

 the local one hinted at by the last speaker. He paralleled the 

 action with the dolomitization of limestones in Ireland, South 

 Wales, and the Isle of Man : cases in which dolomite-crystals ate 



