92 REPORT—1858. 
produce no mineral change in the beds of limestone resting on them, it is inferred 
that the whole group is one formation, which has originated in the simultaneous action 
of aqueous and igneous causes long continued.” Again, he maintains, “that many 
rocks, which now have a perfect porphyritic structure, were so far aqueous deposits, 
that they had been spread out into beds by the action of the sea, and that their actual 
structure (though certainly metamorphic) was not the effect of torrefaction or of 
heat emanating from any eruptive or igneous centre. On the contrary, that the na- 
tural temperature at great depths, combined with great pressure long continued, was 
a cause quite sufficient to explain the phenomena of the old stratified plutonic rocks,” 
The only important modification which I venture to propose in this statement is, 
that we must distinguish between the aqueous causes really simultaneous with the 
igneous ; viz. the water or steam present in the rock when undergoing metamorphic 
changes at great depths by heat and pressure, and from the aqueous causes acting 
long before, when these beds were deposited as true rudimentary strata at the bot- 
tom of the ocean. 
If the positive evidence, then, is strong in favour of these rocks being metamorphic, 
I think the negative evidence is not lessso. In regions where igneous rocks abound 
which are clearly eruptive, we have many characteristic marks of their origin. 
They have a distinct mineral character and structure; they do not graduate into 
slaty rocks. We have distinct evidence of their overflowing neighbouring strata. 
We have distinct evidence of the manner of their eruption from below in the presence 
of many vertical dykes. 
None of these phenomena are observed in the Lake District, as regards the por- 
phyries alternating with the slates. The only difficulty to be accounted for, I think, 
is the great thickness of some of the slaty beds, which have remained unaltered whilst 
other beds above them have been changed by the action of the same heat into por- 
hyry. 
‘ tt & evident, that if the different beds can be shown to be of different degrees of 
fusibility, this cireumstance would be at once accounted for; and I have made some 
experiments to test this point, and also on thie effects of fusion and slow cooling in 
producing metamorphic effects upon rocks. 
The first experiment was made for me by Mr. J. P. Wood, of Leeds, on clay- 
slate, greenstone porphyry, and greenstone roofing-slate, all reduced to fine powder, 
and about } 1b. of each placed in three crucibles; in a common air-furnace, and gra- 
dually heated. At a good red heat the porphyry puffed up, fused, and ran over the 
edge of the crucible in the shape of a brown glassy slag; at a white heat the clay- 
slate fused into a grey glassy slag ; and, lastly, at a strong white heat, the roofing- 
slate also fused into a black glassy slag. Thus the slate rocks appear to be decidedly 
less fusible; less easily acted upon by heat than the porphyries. The same result 
was obtained {in fusing clay-slate, Skiddaw granite, and greenstone porphyry in a 
common reverberatory furnace; the stone being broken into pieces, about the size 
of those used for road-making, a white heat was required to fuse them, but the 
granite and porphyry melted much more readily and completely than the clay-slate. 
These simple experiments may be sufficient to prove a difference in the fusibility of 
the slaty and porphyritic beds, which would account for the latter undergoing changes 
by the same heat which would merely harden the former. 
I next endeavoured to ascertain the effect of pressure combined with heat on the 
fusion of rocks, with the very able and zealous assistance of Messrs. Kitson and 
Hewitson, of this town. These experiments, which are attended with great difficulty, 
and require the patient endurance of a good many partial failures before a success- 
ful result can be obtained, are as yet only in progress. 
Some results, however, may be seen in the specimens on the table. 
The powdered granite, clay-slate, and porphyry were enclosed in strong iron tubes, 
well consolidated to begin with by hydraulic pressure, then screwed up and heated 
to a red heat in an air-furnace, and then slowly cooled. 
The result is a compact mass of distinctly stony texture approaching to slaty 
greenstone, quite different from the porous glassy slags produced by fusion without 
pressure. The fusion has taken place at a lower temperature under pressure, 
