I04 



NATURE 



[May 29, 1890 



was from time to time cracked by the unequal expansion. 

 The solvent, finding its way into the interior, partially 

 dissolved away the original crystal of chrome-alum. The 

 final result of these changes was that while the form of the 

 alum octahedron was almost completely reproduced, only 

 a small portion (a) of the original chrome-alum crystal 

 remained. Much of the chrome-alum was dissolved out 

 and replaced by a mixture of the two alums (b), while 

 the other layers of the crystal (c, D, E, F, g) were 

 formed by still paler-coloured zones, also consisting of 

 mixtures of a like kind. Zoned crystals exhibiting similar 

 abnormal appearances to this alum crystal are by no 

 means rare in some igneous rocks. 



VIII. When solution under pressure is going on in a 

 ays/alline body, the action is controlled and modified by 

 its 7nolecular structwe. This molecular structure may 

 have been produced either in the process of crystallization, 

 or as the result of tnechanical or other forces acting upon 

 the crystal subsequently to its formation. 



Daniell's earliest contributions to science, in the year 

 1816, dealt with the remarkable and unequal action of sol- 

 vents upon crystals. The curious and complicated patterns 

 produced on the faces and the cleavage or cut surfaces of 

 crystals (etching-figures) have subsequently been studied 

 by Leydolt, Klocke, Baumhauer, Becke, and other investi- 

 gators. The results obtained have been shown to vary 

 with the nature and strength of the solvent, the tempera- 

 ture, the pressure, and the time during which the action 

 is allowed to take place. 



In 1884-85, Von Ebner, as the result of an exhaustive 

 study of the etching-figures of calcite and aragonite, 

 showed that crystals possess //<a;«^j of chemical weakness, 

 to which he gave the name of " solution-planes," these 

 being analogous to the well-known planes of least co- 

 hesion or cleavage-planes. Quite independently, I, about 

 the same time, arrived at the same conclusion by study- 

 ing the crystals in deep-seated rocks (Quart. Journ. Geol. 

 Soc, xli. 383, &c.). In these deep-seated rocks the 

 crystals (their whole substance being permeated by the 

 solvent) yield to chemical action along their solution- 

 planes, along which hollow spaces in the form of negative 

 crystals are produced. 



When twinning-planes are developed in crystals by 

 pressure or other mechanical agencies, these planes 

 (gliding-planes) become planes of chemical weakness 

 {Mineralog. Mag., vii. 87). The experiments of Reusch, 

 Baumhauer, Miigge, Foerstner, and others have shown 

 how frequently this secondary twinning is developed in 

 the crystals of rock-forming minerals. 



When the negative crystals formed along the solution- 

 planes of a mineral are filled with various secondary 

 products, the whole character and aspect of the substance 

 may be transformed. When the infilled negative crystals 

 are of appreciable dimensions, the avanturine and 

 " Schiller " phenomena result from the action. When 

 the action is on an ultra-microscopical scale, the phe- 

 nomena of opalescence and of iridescence may be 

 produced. 



By the introduction of various substances in solution 

 into a crystal, its composition may be altered and the 

 way prepared for the recrystallization of the substance as 

 a distinct mineral. It has been shown that, by the intro- 

 duction of sodic chloride into a plagioclase-felspar, the 

 way has been prepared for the conversion of that mineral 

 into scapolite {Mineralog. Mag., viii. 186). 



IX. Under great pressures, paramorphic changes take 

 place in crystalline bodies without any alteration in their 

 chemical composition. 



It is a well-known fact that, under the slight pressure 

 which can be exerted by the hand, the orthorhombic, 

 yellow variety of mercuric iodide passes into the tetra- 

 gonal, red variety. Spring has shown that, under a 

 pressure of 5000 atmospheres, monoclinic sulphur passes, 

 at ordinary temperatures, into the orthorhombic form. 



NO. 1074, VOL. 42] 



Van 't Hoiif and Reicher have shown that the tem- 

 perature at which this latter change takes place is 

 progressively diminished as the pressure is increased. 



That slight forces acting through a considerable period 

 of time are competent to produce such paramorphic 

 changes has long been known. Thus the mercuric iodide 

 and sulphur undergo their paramorphic changes slowly 

 when subjected only to the ordinary vicissitudes of 

 atmospheric temperature. 



Many interesting examples of similar heteromorphous 

 forms of the same compound are familiar to geologists, 

 such as calcite and aragonite among the carbonates, and 

 pyroxenes and amphiboles among the silicates. Hetero- 

 morphism, indeed, appears to be the rule rather than the 

 exception in the mineral kingdom. 



The slow paramorphic changes between hetero- 

 morphous forms of the same compound was long ago 

 studied by Gustav Rose ; and in more recent years the 

 dependence of these changes on great pressures, or on 

 small forces acting through long periods of time, has 

 engaged the attention of J. A. Phillips, Allport, Hawes,. 

 R. D. Irving, J. Lehmann, G. H. Williams, Teall, and 

 other observers. 



In considering these paramorphic changes, it must be 

 remembered that the transition under pressure is not 

 always, as in the case of sulphur, from a less dense to a 

 more dense form. On the contrary, as in the change of 

 both aragonite to calcite and of augite to hornblende, we 

 find the denser but less stable form passing into the less 

 dense but more stable one. Stability, however, is only a 

 relative term : while one form of a compound may be 

 most stable at one temperature or under a certain 

 pressure, other conditions may exist under which it 

 becomes an unstable form. 



X. Both solution aftd the formation of new crystalline 

 compojmds may result from pressure, and these two 

 operations may take place together; in this way more or 

 less complete interchange of ingredients may take place 

 between the crystalline bodies, and pseudomorphs be 



formed. 



That most of the pseudomorphic changes, so common 

 in the mineral kingdom, take place at considerable depths 

 from the surface there seems no room to doubt ; and in 

 all these cases it may be inferred that pressure is one of 

 the determining conditions of the action. 



The effects of these pseudomorphic changes in trans- 

 forming vast rock-masses into others of totally different 

 composition — such as limestone into dolomite, chalybite,^ 

 or silica — has long been familiar to geologists ; and 

 modern microscopical methods have enabled us to trace 

 the progress of these changes from their earliest begin- 

 nings to their complete consummation. 



Without entering further into this very wide question, 

 I may mention that Mr. G. F. Becker has lately published 

 the full details of his studies of the Coast- Ranges of Cali- 

 fornia, and that these tend to prove that, in comparatively 

 recent geological times, vast masses of rock in that dis- 

 trict have had their substance replaced in some cases by 

 silica, and in others by serpentine ; the changes some- 

 times taking place over considerable areas. These con- 

 clusions, arrived at by the officers of the U.S. Geological 

 Survey, if fully established — and there appears to be no 

 room for doubt as to their general accuracy — are not less 

 interesting and suggestive than they are novel and 

 startling. 



XI. When, as the result of dynamical pressures, the 

 crystalline constituents of rocks are brought into close 

 contact, chemical affinity comes into play between them, 

 and new mineral species result from the reactions that 

 take place. This operation is facilitated, when, as a con- 

 sequence of internal strains, differential movements cire 

 set up within the rock-mass, and rubbing or sliding 

 contacts between its particles are brought about. 



Chemists are acquainted with many examples of che- 



