460 Mr. Henwood on the opposite Walls of Cross-veins. 
sitions. In some instances, too, the ingredients of the cross- 
veins and of the adjoining rocks perfectly correspond, both in 
mineral composition and mechanical structure. 
But the prevailing opinion, that the vein intersected must 
be older than that which traverses it*, if still maintained, 
must be received with considerable limitation} : for it not only 
ascribes different ages to neighbouring veins which are exactly 
alike in every respect, on no other evidence than that of their 
intersections (p,q,7), but whenever two veins mutually in- 
tersect each other at different levels (7, s, w, x, y,), this theory 
places its supporters in the dilemma of requiring that one is 
at the same time both older and newer than the other. 
In the pursuit, therefore, of the severed portions of lodes 
which have been heaved, observation and experience are still 
our only guides. 
In all the foregoing investigations{ it has been assumed that 
any motions have been at periods subsequent to that at which 
the rocks became perfectly rigid and incompressible. Indeed 
it would have been vain to have attempted an explanation of 
the observed facts, on purely mechanical principles, under 
any other conditions. 
If we suppose it possible for the rocks to have been so 
broken that each lode was contained in a different fragment, 
and that these minute masses had an independent motion, in 
any direction and to any extent required, although such 
movements might, within certain limits, have afforded any 
desired results, yet the motions in different directions neces- 
sary to the production of the observed phznomena, in dif- 
ferent portions, would often have required that the rocks 
should in some spots have suffered much compression, whilst 
* Dr. Borlase, Nat. Hist. of Cornwall (2nd edit., 1758), p. 152; Mr. 
Pryce, Mineralogia Cornub. (1778), p. 101; M. Werner, Treatise on Mi- 
neral Veins (1791), p.53; Mr. Thomas, Report (1819), p. 21; Mr. Carne, 
Cornwall Geol. Trans. (1819), ii. p. 123; Mr. Hawkins, ibid. p. 232; M 
De la Beche, Report (1839), p. 353} Prof. Phillips, Cab. Cyclo., Geology 
(No. cxi. 1839), ii. p. 136. 
+ Dr. Boase, Primary Geology, p.365; Mr. R. W. Fox, Report of the 
Royal Cornwall Polytechnic Society (1836), p. 120. 
t The principles of simple mechanical displacement here investigated 
were held by M. Schmidt. ‘They have been examined in detail by M. 
Zimmermann, Die Wiederausrichtung verworfener Gange, Lager, und 
Flotze (Leipzig, 1828); more generally by myself, Proceedings of the 
Geological Society (1882), i. p. 406; Mr. Hopkins, Cambridge Phil. Trans. 
(1835) vi. p.58; Mr. Burr, Mining Review (1836), No. vill. p. 236; Mr. 
R. W. Fox, Report of the Royal Cornwall Polytech. Soc. (1836), p. 120 ; 
myself, Edin, New Phil. Journal (1836), xxii. p. 161; and Reports of the 
British Association (1837), p- 74; Mr. De la Beche, Report (1839), p. 298 ; 
Prof. Phillips, Cab, Cyclo., Geology (No. cxi. 1839), ai/p. 145. 
