RESEARCHES OH THE CONTORTION OF ROCKS. 
23 
parison with cast steel. Absolute rigidity is an unknown pro- 
perty of matter. Let. us select a few examples of what are 
commonly regarded as rigid bodies. Eock crystal, glass, calc 
spar, steel and limestone, are surely fair specimens. Yet, 
Tyndall gives instances of quartz crystals altered in shape by 
pressure, some of them having yielded along transverse planes 
as if one-half had slidden over the other, but subsequently 
strongly cemented together by mere apposition and pressure. 
He regards the action of strongly compressed glass upon polar- 
ized light as proof of an alteration in its molecular arrange- 
ment. Mr. Sorby has cited examples of distorted crystals of 
calc spar in cleaved limestones. M. Tresca, in his paper on the 
“ Flow of Solids,” read before the Institution of Mechanical 
Engineers at Paris in 1867, gives the result of experiment^ 
made upon lead, iron, and even steel, and shows that these 
metals behave like liquids when subjected to adequate pres- 
sures. As to limestones and other rocks, I can say from my 
own experiments that they are both elastic and plastic, yielding 
more or less to forces of short duration, but recovering their 
original figure, while when subjected to long-continued pres- 
sures or strains of low intensity they are capable of setting 
permanently in a new shape. 
It is curious to observe how speculation has been misled by 
the notion of absolutely rigid bodies, by the assumption that 
hard rock can exhibit neither an appreciable elasticity nor any 
ductile properties. Sir James Hall, of Dunglass, whom Pro- 
fessor Geikie has lately styled “ the founder of Experimental 
Geology, since it was he who first brought geological specula- 
tion to the test of actual physical experiment,” investigated 
the subject of contortion with much care. He had previously 
carried out laborious inquiries into the influence of pressure in 
modifiying the action of heat. The curved strata of the Ber- 
wickshire coast had engaged his attention since the year 1788. 
In 1814 appeared his remarks on “ The Convolutions of Strata 
and their meeting with Granite.” * In this interesting paper he 
describes the local phenomena with some minuteness, and then 
gives the “ rude experiment ” contrived to imitate the condi- 
tions which he supposed to have obtained in nature : — - 
66 Several pieces of cloth, some linen, some woollen, were 
spread upon a table, one above the other, each piece represent- 
ing a single stratum ; a door (which happened to be off the 
hinges) was then laid above the mass, and being loaded with 
weights, confined it under a considerable pressure. Two boards 
being next applied vertically to the two ends of the stratified 
mass, were forced towards each other by repeated blows of a 
* “ Transactions Boyal Society, Edinburgh,” vol. vii. pt. 1. 
