♦ KNOWLEDGE • 



[June 23, 1882. 



CRYSTALS. 



By William Jaoo, F.C.S., Assoc. Lsst. Cuem. 



ANOTHER group of rocks is tliat in wliiih tlie structure 

 is wholly crystalline; they contain no glassy base, and 

 often the crystals are sulliciently large to be readily ilisccrneil 

 by the naked eye. From the very fact that the crystals are 

 so packed and "crowded, it often follows that their shape is 

 not perfect ; those last formed must of necessity tit them- 

 selves into the spaces left for them. Fig. 1 is a micro- 

 scopic study of a i-ock of this description. This particular 

 specimen at one time formed a part of Cleopatra's Xeedle, 

 and thus possesses some littli' interest additional to that it 

 derives from its geological cliaracter. This rock, usually 

 termed " Syenite," is essentially composed of the minerals 

 felspar and hornblende : the specimen before us also con- 

 •liiis (juartz. 



KiK. 



Hornblende is a mineral of a very dark colour ; and 

 even in thin sections is almost opariue ; but little of it is 

 shown in the figure. There is, however, a small piece in 

 the upper part of the field, represented by very dark cross 

 shading. To the left is seen a space which, save a few 

 Btraight lines, is almost free from markings ; this portion 

 of the rock is quartz. Starting from the Ijottora of the 

 figure, and occupying the whole of tlie centre, is a portion 

 of a crj'stal of felspar, approximately square in section. 

 The felspar of Cleopatra's Needle is of special interest to 

 the geologist from some peculiar markings it shows when 

 viewed by polarised light. These consist of a series of 

 striations, across the crystal (from left to riglit of the 

 figure), but as they are most brilliantly coloured, it is 

 impossible to represent them in a plain black-and-white 

 sketch. To us, who are at present engaged in the study 

 of crj-stal life and growth, there is one lesson in par- 

 ticular the felsfjar crystal teaches us. It may be noticed 

 that the crystal is cloudy, instead of being clear like 

 the quartz ; this cloudiness has developed very fully 

 in the upfx;r right-hand comer of the crystal, where, 

 in the figure, it is repre,8ent^;d by a sort of stippling 

 effect, but the whole crjstal is permeated by it, although 



to a lesser extent From a comparison of ditFerent rocks 

 it is found that the felspar of some are almost clear, 

 while in others the mineral is entirely changed. Those 

 which have been most subjected to the action of " weather- 

 ing," have suft'ered most. Rain, frost, and last, but not 

 least, the atmosphere, gradually do their work, and slowly 

 but surely demolish the crystal edifice, so laboriously built 

 up by Nature in her constructive moods. Felspars are par- 

 ticularly liable to decomposition ; the lime and potash or 

 soda they contain are dissolved out by water containing 

 carbon dioxide in solution, and leave behind a form of sili- 

 cate of alumina, to which the name of kaolin has been 

 given. This substance will be more familiar to many as 

 " china clay." It is to this kaolin that the cloudy nature, 

 of the felspar crystals is due, and in but few rocks is the 

 felspar entirely free from it Not only, then, have we 

 crystal growth and life, but these are followed by crystal 

 decay, and ultimate decomposition. Water, the grand 

 agent of geological denudation, not only does its work in 

 the form of the beating wave and the rushing torrent, but 

 also acts quietly on a gigantic scale on the constituent 

 molecules of rock matter, and reduces them to the line 

 mud and sediment fiom which are again built up our vast 

 form itions of sidnncntary rocks. 



It IS \\ell known that sedimentary rocks, after a time, 

 b((Oiiie much altered in appearance and properties. This 

 IS ji iititui lily noticeable in those strata which, lying deej* 

 li(ii(ath tli( surfue, arc subjected to enormous pressure 

 iiul a tonsideiable degree of heat. Although the rock 

 iiiu not be melted, still the heat favours in a remarkable 

 111 iiiiii 1 the ti ndeiK J of the particles to assume once more;- 

 tile ei)stilline foim 



Fi;,'. -'. Fig. 3. 



Fig. 2 represents a piece of slate from Bavaria, which 

 has been altered l)y the intrusion of some eruptive rocks. 

 Under the heat produced, crystallisation has set in, and 

 the slate contains a ejuantity eif crystals of a mineral known 

 as chiastolite>. The section of one, almost at right-angles, 

 is figured. This particular .specimen tells us much of the 

 growth of the crystal. We notice that it has suce:(^e'eled in 

 thru.sting the grain of the slate aside where it me;t it per- 

 pendicularly, but where the growth has been in the; line of 

 cleavage of the slate, the slate has held its own ; two oppo- 

 site corners of the crystal are thus imperfect Fig. 3 is a 

 ske;tch of rock called " granulite." It is a highly-meta- 

 morphosed, sedimentary rock ; the whole mass is crystalline, 

 the crystals being arranged, however, in re-gular lines, thus 

 furnishing a clue to its origin. The two larger masses 

 drawn are imperfect crystals of garnet — a plentiful mineral' 

 in rocks of this type. 



The rocks have afforded us some striking examples of 

 crystals — tlieir growth and decay. One last lesson they 



