jukes’s manual of geology. 
145 
“ When these superficial bendings of slate occur on steeply inclined ground, they may 
perhaps be referred to the action of gravitation on substances loosened by weathering, or 
the ‘ weight of the hill,’ as it has been called. In other cases their origin is more obscure, 
and I have seen one instance in North Wales, where, on the horizontal surface of an iso¬ 
lated boss of rock, the slates were so sharply and abruptly bent back and laid nearly flat, 
and partly consolidated in that position, as to give the idea of its being due to some 
sudden and great force, such as the grounding of an iceberg. 
“ Thoroughly to work out the subject of the ‘ cleavage’ of any district would require 
months of continuous and laborious observation in a country, the geological structure of 
which had in other respects been thoroughly and accurately surveyed; and with the 
exception, perhaps, of North Wales, no country has yet been surveyed with anything 
like an approach to such accuracy. 
“ It must be recollected that it is one thing to arrive at a conclusion as to the cause 
of cleavage, and the laws of its production, and another to ascertain those laws or general 
rules of occurrence of cleavage planes in nature. The first may be done in the closet, or 
the museum, as has been done by Sorby, Tyndal, and Haughton, but the latter can only 
be done by the field surveyor, and that after and not contemporaneously with the general 
survey of the country.” 
We extract the section on the relations between Felstone and Green¬ 
stone whole, on account of the interest of the view it opens up, and 
also because it is one in which we are not fully prepared to coincide. 
The question is fully discussed in a remarkable paper by M. Durocher, 
a translation of part of which is published in the present Number of our 
Review. 
“I have occasionally been struck in some of the districts just alluded to, with the 
association of felstone and greenstone, it being rare to find any considerable mountain 
mass of felstone without irregular patches of crystalline greenstone disseminated about 
it. The irregular outline of these greenstone patches gave them the appearance of being 
subsequently intrusive into the felstone, but the frequent association of the two has some¬ 
times led me to speculate on the possibility of the two rocks having been part of the same 
molten mass, and having settled or segregated apart from each other on the cooling of 
the whole. There seems no very cogent reason why we should necessarily suppose the 
whole molten mass to have been completely homogeneous; but granting that it was so, 
is it not possible that, when a deep-seated mass of trap commences to cool, a separation 
may take place, and one more fusible portion of it may be segregated from the rest, and 
thus one or more local centres might be established, into which the greater portion of the 
more fusible bases (silicates of lime and iron) should be concentrated ? These local 
patches, which, on the ultimate complete refrigeration of the whole, would form green¬ 
stone, while the rest of the mass was felstone, or elvanite (quartziferous porphyry), as the 
case may be, might retain their fluidity for a time, till, on the consolidation and consequent 
contraction of the other mass, they were squeezed in various directions into the cracks 
and fissures that would then be caused, and then cool rapidly in consequence of their 
greater extent of surface.” 
Again, we find, at page 293, the following remarks, with which we 
only concur in part :■— 
“ If we assume all igneous rocks to proceed either from one central molten mass of 
equable constitution throughout, or from separately fused portions of perfectly similar con¬ 
stitution, might we not suppose that the difference in the constitution of the various pro¬ 
ducts which we find at the surface depended on the circumstances and conditions in which 
they had been placed ? The portions now open to our examination had probably to pass 
through different thicknesses and different kinds of other rocks ; they would be placed then 
under different conditions of temperature and pressure, which might perhaps alone cause a 
separation to take place in their different ingredients; they might also take up in their 
