50 nEPORT — 1853. 



Bos primigenius, of the Cervus Elaphus, Equus Caballus, Capra Hircus ?, and Stis 

 Scrofa. Associated with them are laid horizontally the trunks of trees, as the oak, 

 fir, and others, and hazel-nuts. ^ 



On a Chemical Cause of Change in the Composition of Rocks. 

 By Professor Johnston, M.A., F.R.S. L. ^ E. 



Tlie first example of a chemically altered rock adduced by the Professor, was the 

 rottenstone of Derbyshire, a light and porous substance used chiefly for polishing 

 metals, and stated in Phillips's ' Minei*alogy ' to be composed of silica, alumina, and 

 carbon. It is obtained from a ridge known as the Great Fin, on the right-hand side 

 of the road from Bakewell to Buxton. This ridge is covered with "drift" 10 or 20 

 feet thick, consisting of brown clay, with fragments of black marble, chert, and rotten- 

 stone. The rottenstone is so soft wliilst in the soil that the spade goes through it 

 readily, but it hardens on exposure ; the holes from which it is dug are sometimes 

 only 2 feet deep, at others from G to 8 feet. On examining a series of specimens, Pro- 

 fessor Johnston found that whilst some were homogeneous, others had a nucleus of 

 black marble; he then treated specimens of the black marble with weak acid, and 

 found that on the removal of the carbonate of lime, there remained from 30 to 35 per 

 cent, of a siliceous substance perfectly like the natural rottenstone. He concluded 

 that there existed in the soil some acid which penetrated it and dissolved out the cal- 

 careous matter of the rocks below. The agent in this case might be said to be the 

 carbonic acid of the air brought down by rain ; but there were instances not capable 

 of explanation by this agency alone, and attributable to other acids, which are con- 

 stantly being produced luider certain conditions and exercise a much wider influence. 

 The acids he alluded to were those which are produced naturally and everywhere by 

 the decay of vegetable matter. The bottoms of peat bogs present verj' strong evidence 

 of the action of acids; the stone and clay are bleached and corroded, only siliceous 

 and colourless materials being left. The source of the acid is here the same as in the 

 former instance ; the vegetable matter growing on the surface produces in its decay 

 acid substances which exert a chemical action on the subsoil, and escape by subterra- 

 nean outlets, cai'rying away the materials dissolved in their progress. Another instance 

 ■was aflPorded by the mineral Pigotite, formed in the caves of Cornwall by water dripping 

 from the roof; this water contains a peculiar organic acid, derived from the soil of the 

 moors, which dissolves the alumina of the granite and combines with it. The organic 

 acids are very numerous and differ in composition, but agree in producing chemical 

 action upon rocks. They are produced over the entire surface of the earth, especially 

 over uncultivated tracts, and are among the means pi'ovided by nature to dissolve the 

 mineral food of plants; they are also amongst the chief causes of the exhaustion of 

 soils. The author then alluded to Professor Way's examination of some of the green- 

 sand strata of Surre}', known as firestone, — a light and porous rock, containing 

 silica in a soluble state. It was well known that common sandstone, quartz, or rock 

 crystal were not acted upon by potash or soda at ordinary temperatures ; but of the 

 firestone 30 per cent., and sometimes .50 or 70 per cent., may be dissolved. In all 

 such cases the silica must have been originally in a state of chemical combination 

 ■with lime, alumina, or something else, which has been subsequently removed. The 

 silica in the rottenstone was soluble, but he had never met with instances of black 

 marble in a bedded state converted into rottenstone. He believed, however, that a 

 similar cause, operating over a wide area, and during a long period, had produced the 

 altered condition of the firestone. Professor Johnston then alluded to the nodules of 

 phosphate of lime in the greensand and crag, and suggested that the phosphorus had 

 been derived from animal remains in higher strata, dissolved out by acids, and re- 

 deposited at a lower level. The last example was the Jireclay of the coal-measures, 

 8 stratum almost imiversally found beneath beds of coal. It differs from the other 

 clays both in colour and composition, being whiter, and containing less of those sub- 

 stances which acid bodies could dissolve, viz. the earthy bases, which would render 

 the clay fusible in fire ; the condition of the fireclay might be accounted for by the 

 action of acids developed during the production of the vegetable matter now forming 

 coal. 



