4i6 



NA TURE 



[March 3, 1887 



a crystal. It thus appears that the comphcated etched 

 figures, with their curved and striated surfaces, are indi- 

 cations of the combination or oscillation of tendencies 

 to chemical action along the different solvent-planes of 

 the crystal. 



My own experiments have enabled me to show that the 

 chemical action taking place along the solution-planes of 

 crystals leads to the development of cavities, often 

 assuming the forms of negative crystals, which may 

 become wholly or partially filled with the product of the 

 chemical action. 



Although the solution-planes are quite distinct, both 

 from the gliding-planes and the cleavage-planes of 

 crystals, I have been able to show that some curious and 

 interesting relations exist between them. If lamellar 

 twinning has been already developed in a crystal, then 

 chemical action takes place along the gliding-planes in 

 preference to the normal solution-planes. 



It is only when we study the minerals building up the 

 rock-masses of the globe that we fully realise the import- 

 ance of these molecular structures, and the wonderful 

 changes which crystals are capable of undergoing, as a 

 consequence of their internal " organisation." Then, and 

 then only, do we begin to understand the significance 

 and the far-reaching consequences of the physiological 

 changes of which minerals are susceptible. 



The crystals forming the rock-masses of the globe 

 have been subjected to every variety of mechanical force 

 — violent fracture, long-continued strain, steady but 

 enormous pressure — prolonged over vast intervals of time, 

 to which must be added the potent effects of alternate 

 heating and cooling. Such crystals, moreover, are trans- 

 fused through their whole substance by various liquids 

 and gases acting under tremendous, and sometimes vary- 

 ing, pressures. 



Under such circumstances it is not surprising to find 

 that the crystals have often yielded along their cleavage- 

 planes, and that cleavage-cracks have been produced. 

 These, by affording a ready channel for the passage of 

 solvents, not unfrequently determine the course of various 

 chemical operations going on within the crystal. 



Not unfrequently, too, the rock-forming minerals have 

 yielded along their gliding-planes, and the development 

 in thern of twin-lamellc-e is the result. Every crystal of 

 calcite in an ordinary metamorphic limestone, and many 

 of the plagioclase feldspars in igneous rocks, exhibit the 

 secondary lamellar twinning which has arisen from the 

 action of mechanical forces upon the mass.i The 

 rnicrocline structure in orthoclases, with many other 

 similar structures in other minerals, must almost certainly 

 be ascribed to the same cause. 



Still more remarkable are the consequences which 

 follow from the existence of the solution-planes in crystals. 

 By the action of various solvents under pressure, augite is 

 made to assume the forms known as diallage and pseudo- 

 hypersthene, the ferriferous enstatite of bronzite or 

 hypersthene, while the feldspars acquire their avanturine, 

 Schiller, and chatoyant phenomena. When, in addition 

 to the statical pressures due to thousands of feet of super- 

 incumbent rocks, these solvent agencies work with those 

 tremendous dynamical aids afforded by deforming stresses, 

 such as make the rocks to flow during mountain-making, 



^ It has often been asseited that the "striation" on the faces or cleavage- 

 surfaces of crystals is an indication of the existence of polysynthetic twin- 

 ning. But in the oligoclase ofYtterby and other localities, I have found 

 that many crystals which exhibit striation do not affect polarised light 

 differently in the alternate stria:. But on submitting the crystals to 

 alternate heating and cooling, and sometimes by percussive force, the twin- 

 ning may be easily developed in them. It appears from these observations 

 that the crystals are built up of lamella;, in which the molecules are 

 alternately in stable and unstable equilibrium. I have in some cases found 

 that the stresses upon a slice of feldspar which is being heated and cooled 

 and then ground into a thin section, while cemented to a glass plate during 

 the preparation of a microscopic slide, are sufficient to cause the rotation of 

 the molecules in the alternate lamellje. In some cases, I have no doubt that 

 twin-lamellalion, l.ke cleavage-cracks, may be induced in the crystals of our 

 rock-sections during the processes to which they are submitted in their 

 preparation. 



it is not surprising to find the molecules of the original 

 crystals breaking from their old allegiances, and the 

 liberated atoms uniting to form new minerals, the 

 position of which is determined by the lines of flow in 

 the mass. 



Not a few of our gems owe their exquisite beauties to 

 these physiological changes which have taken place in 

 them since their first formation. The ardent glow of the 

 sunstone and the pale watery gleam of the moonstone, no 

 less than the lovely play of the azure tints in Labrador-spar 

 and the bronzy sheen of Paulite, are the result of phy- 

 siological processes taking place in crystals which were 

 originally clear and translucent. In the profound labora- 

 tories of our earth's crust slow physical and chemical 

 operations, resulting from the interaction between the 

 crystal, with its wonderful molecular structure, and the 

 external agencies which environ it, have given rise to 

 new structures, too minute, it may be, to be traced by 

 our microscopes, but capable of so playing with the 

 light-waves as to startle us with new beauties, and to add 

 another to 



" The fairy tales of science, and the long results of time." 



Yes ! minerals all have a life-history, one which is in 

 part determined by their original constitution, and in part 

 by the long series of slowly-varying conditions to which 

 they have since been subjected. In spite of the circum- 

 stance that their cycles of change have extended over 

 periods measured by millions of years, the nature of their 

 metamorphoses and the processes by which these have 

 been brought about are, in all essential respects, analogous 

 to those which take place in a Sequoia or a butterfly. In 

 spite, too, of the limitations placed upon us by our brief 

 existence on the globe, it is ours to follow in all its com- 

 plicated sequence this procession of events, to discover 

 the delicate organisation in which they originate, to deter- 

 mine the varied conditions by which they have been 

 controlled, and to assign to each of them the part which 

 it has played in the wonderful history of our globe during 

 the countless ages of the past. 



The subject of distribution, or chorology, is one of no 

 less importance in the study of the mineral than in that 

 of the vegetable and animal kingdoms. The relations of 

 minerals to one another, and the manner in which they 

 make their appearance in respect both to time and place, 

 constitute a most instructive and suggestive field of 

 research. 



The older mineralogists paid some attention to the ques- 

 tion of the mode of association of minerals with one another, 

 which they described under the term "paragenesis." But 

 this was at a time when only large and freely crystallised 

 specimens received much attention. At the present day 

 this question of the varied distribution of minerals in space 

 and time, and the manner in which they are associated 

 with one another to build up rock-masses, constitutes a 

 most important branch of our science, that to which the 

 name of petrology is given. 



Under the name of " petrography " an attempt has been 

 made to establish a branch of natural-history science 

 which shall bear the same relation to mineralogy as that 

 science does to chemistry. As minerals are formed by 

 the union of certain chemical compounds, so rocks, it is 

 argued, may be regarded as being built up of diflerent 

 minerals. But it must be remembered that while minerals 

 possess a distinct individuality — the result of their different 

 chemical constitution and their characteristic crystallo- 

 graphic form — we are quite unable to point to anything 

 analogous to these in the case of rocks. 



How is a rock-'' species " to be defined? It is not 

 enough to state its ultimate chemical composition ; for 

 rocks of the most varied character and origin may agree 

 in this respect. Equally futile is it to take mineralogical 

 constitution as the basis of our classification ; for, in the 



