20 



KNOWLEDGE 



[Juke 9, 



formed, because the almost imperceptible rate at which, in 

 some cases, they have l>een laid down has an important 

 liearing on the age of the contents imbedded within them. 

 The surface-wat<»r tinds its way through some crevice or 

 tissure in the rock, and being moi-e or less charged with 

 carlwnic acid deri\ed from the atmosphere, and from 

 decayed vegetable and other matter in the soil, becomes a 

 very powerful solvent, under which the rock, converted 

 into carlionate of lime, is carried away in solution. But 

 while much thus runs off, Nature, true to her cyclical 

 action, begins to refill the cavity which she has eaten 

 away. Drops of water, holding the lime in solution, ooze 

 from the roof, and, falling on the floor, gradually form 

 beds and l)Osses of stjilagmito, wliich hermetically seal 

 whatever de'iris may happen to be lying about, while such 

 portions of the dissolved limestone as remain clinging to 

 the roof after evaporation of the water from them hang 

 icicle-like therefrom, and gradually form the beautiful 

 columns of stalactite which adorn many of the famous 

 caverns of the world. 



CRYSTALS. 



Bv William Jago, F.C.S., Assoc. Inst. Cixem. 

 No. III. 



IN our last paper we studied crystals, artificially pro- 

 pared, under the microscope ; it is our purpose in this 

 numljer, still using the microscope as our means of investi- 

 gation, to examine crystals which have been formed natu- 

 rally. From what has already been stated, it will be seen 

 how universal is the tendency which matter has, when 

 assuming the solid state, to also become crystalline. 

 Rocks form no exception to this rule, and in many speci- 

 mens of the granitic type, large and well- formed crystals are 

 found embedded in the finer portions of the rock. The 

 name "porphyry" is now restricted by geologists to rocks 

 of this class. The stone of the large polished granite 

 fountain in St. Paul's Churchyard contains some very fine 

 crj-stals of the minerals felspar and hornblende ; but not 

 only is granite distinctly made up of crystals, but even the 

 finer grained rocks, which to the naked eye appear per- 

 fectly uniform in texture, are also found on minute exami- 

 nation to be more or less crystalline in structure. Of 

 recent years the application of the microscope in geology 

 has l)een much extended, and now the description of any 

 rock would not be considered complet<; without mention 

 of its microscopic characteristics. To examine rocks with 

 the microscope, by means of transmitted light, in the way 

 most usually adoptf;d, it is neces.sary to prepare and mount 

 extremely thin sections, these sections being often much 

 thinner than paper. At pre.sent we shall not be able to 

 give directions for the preparation of such specimens, but, 

 with the Editor's permi.ssion, may make practical direc- 

 tions for section-grinding and mounting the subject of a 

 .separate paper on some future occasion. 



In the first paper of this series it was stated that the 

 black, pitch like form of lava is instanced by the geologist 

 as the type of glas.sy or non-crystalline rocks. Fig. 1 is a 

 sketch made of such a piece of gla.ssy lava, "obsidian" 

 from the Lipari Isles, magnififid to about 2.^0 diameters. 

 It i.s almost a pure natural glass; the small black specks 

 are probably embryonic crystals of magnetite (one of the 

 natural oxides of iron). Such a rock is produced by the 

 rapid cooling and golidilication of molt<in rock material ; 

 with slower cooling, "obsidians" are formed in which a 

 decided step further in advance towards crystallisation is 



made. Fig. "2 represents a spherulitic obsidian from Ice- 

 land. (This, and all the succeeding figures, are n\agnifieu 

 from abnut '2"> to 30 diameters.) 



Although the main portion of the rock is glassy, there 

 are yet a few small crystals to be noticed ; the most 

 interesting point, however, about the specimen is that it 

 contains a number of small spherules, one of which occu- 

 pies the centre of the field. It may be noticed that it has 

 a radiated structure. Precisely the same formation is 

 often seen in glass which has been maintained at a tempera- 

 ture near its melting-point for a considerable time. The 

 writer has several interesting examples of such glass in 

 his possession. But, to proceed to some further proof of 

 the crystalline nature of these spherules. Students of 

 physics are aware that a ray of light undergoes a peculiar 

 change on passing through a block of Iceland spar ; 

 among other modifications, it is so altered that it is incap- 

 able of passing through a second block of the same material 

 if placed in a certain position with regard to the first. The 

 light under these circumstances is said to he polarised. Micro- 

 scopes for the study of rock sections are fitted with a pair 

 of specially-prepared pieces of Iceland spar, called " Nicol's 

 Prisms." One of these is fixed under the stage ; the other 

 is either fitted over the eyepiece, or into the body of the 

 instrument. These being placed in position, on rotating 

 one of them, a point is reached at wliich, on looking through 

 the instrument, it is seen that no light is transmitted. 

 ]Most crystalline substances, on being placed between the 

 prisms, possess the property of so modifying the beam of 

 light as to effect its passage through the second or upper 

 prism, and thus to the eye, neglecting for the time being 

 those crystals inactive in this respect. This aflTords us a 

 most ready means of discriminating between crystals and 

 glass. The glass of the microscopic slide has no action on 

 the light, neither has a purely glas.sy piece of rock ; but let 

 th(!re be crystals present, and they are immediately seen, 

 ilhiMiiiKated on a black ground. Viewed in this way by 

 polarised light, the darker spherules of the obsidian Ijecome 

 Ijright, wliile the rest of the rock appears black.' 



Leaving the glassy rocks, we may now glance at some 

 specimens in wliich crystallisation has still further asserted 

 itself. Th(! rock called Felsite is one which, instead of 

 being glassy, is micro-crystalline in structure : the crystals 

 are very small, but their optical behaviour leaves no doubt 

 as to their natur(^ There are many rocks which contain, 

 imbedded in a base of this kind, larger, but. still microscopic, 

 crystals. Figs. .3 and 4 are examples of such. The 

 crystals, though so' small, are often wonderfully perfect. 



Fig. 3 is a sketch of a micro-porphyritic basalt, from 

 Unkel, on the Rhine ; the largest crystal occupying the 

 centre of the field is one of felspar, several smaller crystals 

 of the same shape may be detected, the remainder of the 

 work being micro-crystalline. The next figure gives a 



