Ch.XXVi.] ALTERED ROCKS. 373 



affected, it is not only that it has acted less on those, but that, 

 if it had equally affected them, they never could have existed, 

 or would have been all granitic and venous gneiss *. 



According to these views, gneiss and mica-schist may be 

 nothing more than micaceous and argillaceous sandstones altered 

 by heat, and certainly, in their mode of stratification and lami- 

 nation, they correspond most exactly. Granular quartz may 

 have been derived from siliceous sandstone, compact quartz from 

 the same. Clay- slate may be altered shale, and shale appears to 

 be clay which has been subjected to great pressure. Granular 

 marble has probably originated in the form of ordinary lime- 

 stone, having in many instances been replete with shells and 

 corals now obliterated, while calcareous sands and marls have 

 been changed into impure crystalline limestones. 



Associated with the rocks termed primary we meet with 

 anthracite, just as we find beds of coal in sedimentary for- 

 mations, and we know that, in the vicinity of some trap dikes, 

 coal is converted into anthracite. ' Hornblende schist,' says 

 Dr. Macculloch, ' may at first have been mere clay, for clay or 

 shale is found altered by trap into Lydian stone, a substance 

 differing from hornblende-schist almost solely in compactness 

 and uniformity of texture f.' ' In Shetland,' remarks the same 

 author, ( argillaceous schist (or clay-slate), when in contact 

 with granite, is sometimes converted into hornblende-schist, 

 the schist becoming first siliceous, and ultimately, at the 

 contact, hornblende-schist J.' 



This theory, if confirmed by observation and experiment, 

 may enable us to account for the high position in the series 

 usually held by clay slate relatively to hornblende-schist, as also 

 to gneiss and mica-schist, which so commonly alternate with 

 hornblende-schist. For we must suppose the heat which alters 

 the strata to proceed, in almost all cases, from below upwards, 

 and to act with greatest intensity on the inferior strata. If, 

 therefore, several sets of argillaceous strata or shales be super- 

 imposed upon each other in a vertical series of beds in the same 

 * Syst. of Geol., vol. ii. p. 145. f Ibid., vol. i. p. 210. J Ibid., p. 21 1 



