Ch.XXVL] 
ALTERED ROCKS. 
373 
affected, it is not only that it lias 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.' c In Shetland,' remarks the same 
author, f 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 
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 Geo!., vol. ii. p. 145. f Ibid., vol. i. p. 210. * Ibid,, \\ 211 
