ON SLATY CLEAVAGE AND ALLIED ROCK-STRUCTURES. 849 
attendant phenomena, would doubtless be the first effect of lateral 
pressure, while a more intense application of stress might induce mineral 
changes of the nature already alluded to. Thus it may, according to some 
speculators, be possible to bridge over the differences between schists and 
_ gneisses of certain types, on the one hand, and satiny slates or phyllites 
on the other, while between the latter and ordinary slates every gradation 
may be found. 
In this way we arrive at Dr. Darwin's! theory that ‘in most cases 
foliation and cleavage are parts of the same process; in cleavage there 
being only an incipient separation of the constituent minerals ; in foliation 
a much more complete separation and crystallisation ;’ or, again,” ‘ that 
the same power which has impressed on the slate its fissile structure or 
cleavage has tended to modify its mineralogical character in parallel 
planes.’ This may be compared with the hypothesis of Mr. W. Hopkins,? 
that slaty cleavage is the result of molecular forces, but has its direction 
determined by mechanical stress. Dr. Darwin apparently supposed that 
the mechanical force was not only a directing influence, but also the 
prime cause of the molecular actions. In so far as this is a return to the 
purely crystalline theory of slaty cleavage, as advocated by Professor 
Sedgwick, the mass of evidence referred to in the preceding sections must 
compel us to pass it over; but we may reconcile Dr. Darwin’s valuable 
observations with the theories of Dr. Sorby and others, by admitting that 
many of the rocks which we call slate have experienced a development of 
new minerals (such as micas, chlorites, and epidotes) concurrently with 
the production in them of the cleavage structure, and that there appears 
to be a passage from such rocks into mica-schist and foliated gneiss. In 
fact, it seems probable that if the term cleavage be applied only to rocks 
in which no mineral changes are to be detected, the class of cleaved rocks 
will be much reduced in size; and, further, that if we ascribe all such 
mineral changes in slates to subsequent foliation on cleavage planes, the 
number of our metamorphic regions will be greatly augmented. In North 
Wales, for instance, Professor Sedgwick‘ described the cleavage planes 
of the slate as ‘coated over with chlorite and semicrystalline matter, 
which not only merely define the planes in question, but strike in parallel 
flakes through the whole mass of the rock.’ From the microscopic 
examination of the same rocks, Dr. Sorby ° regarded the minute scales of 
mica which make up a large part of the mass, and resemble in structure 
“pseudomorphs of mica or chlorite after felspar’ as being of secondary 
origin ; and although he apparently considered the origin of these scales 
anterior to the cleavage structure, he stated that these so-called clay 
slates are ‘analogous to very fine-grained mica-schist, into which they 
gradually pass by the increase in the size of the crystals of mica.’ In the 
roofing slates of other districts there is often evidence of mineral changes 
which appear to have been contemporary with the cleavage. 
In leaving this part of the subject, we may remark that certain other 
theories, not otherwise of much interest, have linked together cleavage 
and foliation under a common explanation, Professor H. D. Rogers,® 
1 Geol. Obs. in South America, p. 166 (1846); 2nd ed. p. 466 (1876). 
* Ibid. p. 163; 2nd ed. p. 462. 
$ Camb. Phil. Trans., vol. viii. p. 456 (1849). Cf. also the cautious statement of 
Mr. Jukes quoted in the first section of the present paper. 
4 Geol. Trans., ser. 2, vol. iii. p. 471 (1835). 
** Brit. Assoc. Rep., 1857, Trans. Sect., p. 92. 
C cea Rey. Soc. Edinb., vol. xxi. p. 471 (1856). 
