Chalk and Flint Formation. 35 



calcareous stalactites, which indicate their progres- 

 sive manner of formation. Whether the two other 

 special forms of chalcedony have had anything to do 

 with the ebullitions of the substance in a state of 

 incandescent semi-fluidity, may still be open to 

 inquiry. Agates again, which consist largely of 

 concentric chalcedonic bands, are found to have been 

 abundantly formed in rocks of igneous origin, though 

 they have of late been a good deal speculated on as 

 of aqueous formation. 4 They, also, are foreign to 

 the question of the origin of the flint and chalk 

 formation : and quartz, in like manner, has been 

 excluded from the present inquiry. But a few 

 particulars may here be added as to some siliceous 

 petrifactions which, though long known, seem to 

 require further investigation and illustration. The 

 late Mr. Evan Hopkins 5 tells us that in Peru and 

 Chili pieces of wood that for years had been left 

 standing in old mines have been found partially 

 converted into siliceous fossils, and others again 

 covered only with calcareous spar, metallic silver, 

 grey and red silver ores, and fine crystals of iron- 

 pyrites, by solutions from metalliferous rocks. 

 " Siliceous springs," he writes, " are equally abun- 



4 Some very interesting information on the subject of aqueous 

 silica will be found in the Cantor Lectures of Professor BarfT, on 

 Silica and Silicates, published in the Journal of the Society of Arts. 

 See particularly Lect. ii., in the Journal of August 16th, 1872. 

 The lectures have since been published separately. A curious 

 interchangeable action of silicic acid and carbonic acid was 

 experimentally illustrated by him, and is alluded to by him as 

 follows: "You see here the silicic acid has been able to turn out 

 the carbonic acid from the carbonate of potash at a high temperature ; 

 and before we part, I will show you the reverse, that, in the cold, 

 carbonic ^acid is able to turn out silicic acid." 



5 " On the Connection of Geology and Terrestrial Magnetism," by 

 Evan Hopkins, C.E., F.G.S., 2nd Edit, 1851, pp. 39, 81. 



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