May 28, 1891] 



NATURE 



85 



liarities in crystal growth might be induced by mutilation/ 

 have been asserted by other physicists and chemists not 

 to justify the startling conclusions drawn from them at 

 the time. It must be admitted that new experiments 

 bearing on this interesting question are, at the present 

 time greatly needed. 



In 1 881, Loir demonstrated two very important facts 

 with regard to growing crystals of alum {Compt. rend., 

 Bd. xcii. p. 1 166), First, that if the injuries in such a 

 crystal be not too deep, it does not resume growth over 

 its general surface until those injuries haveb'een repaired. 

 Secondly, that the injured surfaces of crystals grow more 

 rapidly than natural faces. This was proved by placing 

 artificially-cut octahedra and natural crystals of the same 

 size in a solution, and comparing their weight after a 

 certain time had elapsed. 



The important results of this capacity of crystals for 

 undergoing healing and enlargement, and their applica- 

 tion to the explanation of interesting geological pheno- 

 mena has been pointed out by many authors. Sorby has 

 shown that, in the so-called crystalline sand-grains, we 

 have broken and worn crystals of quartz, which, after 

 many vicissitudes and the lapse of millions of years, have 

 grown again and been enveloped in a newly formed 

 quartz-crystal. Bonney has shown how the same pheno- 

 mena are exhibited in the case of mica, Becke and 

 Whitman Cross in the case of hornblende, and Merrill 

 in the case of augite. In the felspars of certain rocks it 

 has been proved that crystals that have been rounded, 

 cracked, corroded, and internally altered — which have, in 

 short, suffered both mechanical and chemical injuries — 

 may be repaired and enlarged with material that differs 

 considerably in chemical composition from the original 

 crystal. 



It is impossible to avoid a comparison between these 

 phenomena of the inorganic world and those so familiar 

 to the biologist. It is only in the lowest forms of animal 

 life that we find an unlimited power of repairing injuries : 

 in the Rhizopods and some other groups a small fragment 

 may grow into a perfect organism. In plants the same 

 phenomenon is exhibited much more commonly, and in 

 forms belonging to groups high up in the vegetable series. 

 Thus, parts of a plant, such as buds, bulbs, slips, and 

 grafts, may— sometimes after a long interval — be made to 

 grow up into new and perfect individuals. But in the 

 mineral kingdom we find the same principle carried to a 

 much farther extent. We know, in fact, no limit to the 

 minuteness of fragments which may, under favourable 

 conditions, grow into perfect crystals — no bounds as to 

 the time during which the crystalline growth may be 

 suspended in the case of any particular individual. 



The next property of crystals which I must illustrate, in 

 order to explain the particular case to which I am calling 

 your attention to-night, is the following : — 



Two crystals of totally different substances may be 

 developed within the space bounded by certain planes, 

 becoming almost inextricably intergrown, though each 

 retains its distinct individuality. 



This property is a consequence of the fact that the 

 substance of a crystal is not necessarily continuous within 

 the space inclosed by its bounding planes. Crystals often 

 exhibit cavities filled with air and other foreign substances. 

 In the calcite crystals found in the Fontainebleau sand- 

 stone, less than 40 per cent, of their mass consists of 

 calcic carbonate, while more than 60 per cent, is made 

 up of grains of quartz-sand, caught up during crystalliza- 

 tion. 



' IVien. Sits. Ber.. xxxix., i860, pp. 611-22; KrdmMn, /oi/rn. Prai-i. 

 Chein., Ixxxi. pp. 356-62 ; VVien. Geol. Verhandl., xii. pp. 212-ij, &c. ; 

 Frankenheim, Pogg. Ann., cxiii . 1861. Compare Fr. Scharff, Pogg. Ann., 

 cix., i860, pp. 529-38 ; Neues Jahrb. filr Min., &c , 1876, p. 34 ; and W. 

 Sauber, Liebig Ann., cxxiv., 1862, pp. 78-82 ; alsoW. Ottwald, "Lehrbuch 

 d. Allg.: Chera.," 1885, i)d. i. p. 738; and O. Lehmann, " Molekular 

 Physik.,"i888, Bd. 



NO. 



p. 312. 

 26, VOL. 44] 



In the rock called "graphic granite," we have the 

 minerals orthoclase and quartz intergro^n in such a way 

 that the more or less isolated parts of each can be shown, 

 by their optical characters, to be parts of great mutually 

 interpenetrant crystals. Similar relations are shown in 

 the so-called micro-graphic or micro-pegmatitic inter- 

 growths of the same minerals which are so beautifully 

 exhibited in the rock under our consideration this 

 evening. 



There is still another property of crystals that must 

 be kept in mind, if we would explain the phenomena 

 exhibited by this interesting rock: — 



A crystal may undergo the most profound internal 

 changes, and these may lead to great modifications of the 

 optical and other physical properties of the mineral; yet, 

 so long as a small — often a very small— proportion of its 

 molecules retnain intact, the crystal may retain, not only 

 its outward form, but its capacity for growing and 

 repairing injuries. 



Crystals, like ourselves, grow old. Not only do they 

 suffer from external injuries, mechanical fractures, and 

 chemical corrosion, but from actions which affect the 

 whole of their internal structure. Under the influence 

 of the great pressures in the earth's crust, the minerals 

 of deep-seated rocks are completely permeated by fluids 

 which chemically react upon them. In this way, negative 

 crystals are formed in their substance (similar to the 

 beautiful " ice-flowers " which are formed when a block 

 of ice is traversed by a beam from the sun or an electric 

 lamp), and these become filled with secondary products. 

 As the result of this action, minerals, once perfectly 

 clear and translucent, have acquired cloudy, opalescent, 

 iridescent, avanturine, and " schiller " characters ; and 

 minerals, thus modified, abound in the rocks that have 

 at any period of their history been deep-seated. As the 

 destruction of their internal structure goes on, the crystals 

 gradually lose more and more of their distinctive optical 

 and their physical properties, retaining, however, their 

 external form ; till at last, when the last of the original 

 molecules is transformed or replaced by others, they 

 pass into those mineral corpses known to us as " pseudo- 

 morphs." 



But while crystals resemble ourselves in " growing 

 old," and, at last, undergoing dissolution, they exhibit 

 the remarkable power of growing young again, which 

 we, alas ! never do. This is in consequence of the 

 following remarkable attribute of crystalline structures : — 



// does not matter how far internal change and dis- 

 integration may have gone on in a crystal — if only a 

 certain small proportion of the unaltered molecules re- 

 main, the crystal mccy renew its youth and resume its 

 growth. 



When old and much-altered crystals begin to grow 

 again, the newly-formed material exhibits none of those 

 marks of " senility " to which I have referred. The sand- 

 grains that have been battered and worn into microscopic 

 pebbles, and have been rendered cloudy by the develop- 

 ment of millions of secondary fluid cavities, may have 

 clear and fresh quartz deposited upon them to form 

 crystals with exquisitely perfect faces and angles. The 

 white, clouded, and altered felspar-crystals may be enve- 

 loped by a zone of clear and transparent material, which 

 has been added millions of years after the first formation 

 and the subsequent alteration of the original crystal. 



We are now in a position to explain the particular case 

 which I have thought of sufficient interest to claim your 

 attention to-night. 



In the Island of Mull, in the Inner Hebrides, there 

 exist masses of granite of Tertiary age, which are of very 

 great interest to the geologist and mineralogist. In many 

 places this granite exhibits beautiful illustrations of the 

 curious intergrowths of quartz and felspar, of which I have 



