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POPULAR SCIENCE REVIEW. 
mallet applied horizontally. The consequence was, that the 
extremities were brought nearer to each other, the heavy door 
was gradually raised, and the strata were constrained to assume 
folds, bent up and down, which very much resembled the con- 
voluted beds of killas, as exhibited in the crags of Fast Castle, 
and illustrated the theory of their formation. 
“ I now exhibit to the Society a machine by which a set of 
pliable beds of clay are pressed together, so as to produce the 
same general effect ; and I trust that the forms thus obtained 
will be found, by gentlemen accustomed to see such rocks, to 
bear a tolerable resemblance to those of nature.” 
The positions which we may now consider to have been estab- 
lished by Sir James Hall’s experiments and reflections are these : 
That strata originally horizontal have been curved and folded ; 
and that the disturbing force has acted in a horizontal direction. 
His further decision that the force concerned is necessarily vol- 
canic may be questioned. The absence of superficial traces of 
volcanic agency over large areas of contorted strata — the lime- 
stone district of Craven, for example — is not easily reconciled 
with the views derived by Hall from his instructor, Hutton. 
We must also emphatically dissent from his tacit assumption 
that the contorted rocks must have been “ in a soft but tough 
and ductile state.” Distorted fossils, crystals, and pebbles can- 
not well have been soft when they were pinched and bent out 
of shape. Nor need we assume such a condition during the 
formation of ordinary curved strata. The mechanical proper- 
ties of limestones and other rocks, dry and at ordinary tem- 
peratures, are such as in themselves satisfy the conditions of 
the problem. 
It is natural that early experimenters should fail to perceive 
many important aspects of the questions which they propose to 
themselves. A highly interesting addition to Sir James Hall’s 
researches on the influence of pressure as modifying the action 
of heat was made by Faraday, who showed that the pressure of 
fifty atmospheres, believed by Hall to be requisite to prevent 
the escape of carbonic acid from limestone during the process 
of crystallization by heat, is not indispensable. The composi- 
tion of the surrounding gas affects the facility of dissipation, 
and in an atmosphere of carbonic acid, fragments of limestone 
may be crystallized by heat at standard pressure. The student 
of geology will not need to be reminded of the importance of 
this qualifying consideration in connection with calcareous de- 
posits in recent volcanic rocks. Similarly, in his contortion 
experiments, Hall stopped short at an early stage. It does not 
appear to have occurred to him that pieces of the very rocks 
whose curvature he was investigating might be made to bend 
permanently by means of the same apparatus employed in 
