CAL OA KH SICONGRMELIONS OLN KERIEE POLNT. (145 
of minerals like wavellite, natrolite, and other zeolites, gypsum, 
serpentine, talc, and other hydrous secondary minerals ; possibly 
pseudomorphs in every case, and if so, of less density, and occu- 
pying greater volume than the parent mineral. Such swelling 
substance can exert active centrifugal force. But we have 
already noted the fact that this cause of crowding in rock-masses 
cannot aid us greatly in explaining the Kettle Point con- 
cretions. 
Basing his statement on such doubtful examples as those just 
mentioned, Bischof says: ‘Crystallization is a force which may 
be compared with that of the expansive force of heat.’’? On the 
other hand, he quotes Kopp, who opposed Duvernoy in his theory 
of a mechanical energy of crystallization by showing that a crys- 
tal of alum growing in a vessel never does so by accretion on 
the face upon which the crystal rests at the bottom of the vessel. 
Kopp thus concluded that the mechanical energy of crystalliza- 
tion must be very slight, if existent at all, in this case, of a crystal 
of exceptional rapidity of growth that cannot overcome its own 
trifling weight when immersed in the mother-liquor.* 
At the same time, certain observers have noted instances 
where live force seems to have been exerted during growth by 
crystals or crystalline aggregates, which may not, or, indeed, 
certainly have not, been pseudomorphous derivatives from pre- 
existing minerals. De La Béche, speaking of crystalline con- 
cretions of selenite and of iron pyrites, stated his belief that, in 
these cases, chemical affinity was strong enough ‘to overcome 
the attraction of cohesion” in the matrix.* Dana’s example of 
rifting of quartzite by the growth of a limonitic deposit, and the 
wedging asunder of parts of a tourmaline crystal by the crys- 
tallization of quartz in which the tourmaline lies embedded, are 
too well known to need more than mention.3 Similarly, Worthen’s 
disjointed crinoids, the plates of which were gradually separated 
by the deposition of quartz between them, are cited by Dana as 
1 BiscHoF, Lehrbuch, Band I, p. 134. 
2 Researches in Theoretical Geology, 1834, p. 91. 
3 Manual of Geology, 4th ed., p. 138. 
