102 



HARDWICKE'S SCIENCE- GOSSIP. 



rock is composed ; the larger opaque crystals, 

 whether white or pink, are felspar, the glassy 

 mineral is quartz, and the little glittering spangles 

 are mica. We may next proceed to a more detailed 

 examination of each of these in turn. We will first 

 ask the chemist what he can tell us of their com- 

 position. The chemist is not satisfied with merely 

 knowing that a certain mineral occurring in certain 

 definite crystalline or other forms is quartz, another 

 felspar, and so on ; but he asks further, — What is 

 this quartz ? Is it a simple body or is it, simple as it 

 may appear to sight, a compound of two or more 

 elements ? He takes various specimens of quartz, 

 some perhaps from the granite, others from some 

 other rocks, and subjects them to the analytical 

 processes of the laboratory : the result is that he 

 finds all quartz, no matter what its colour may be, 

 whether white or pink or black, or pure and colour- 

 less as glass, to be a compound of the metalloid 

 silicon and the gas oxygen ; in other words, that it 

 is an oxide of silicon, to which he assigns the name 

 silica. By a series of analyses he is able to correlate 

 the quartz of the granite with all other forms, and 

 they are many in which this mineral occurs. The 

 flint of the chalk, the white veins so often met with 

 in the older slaty rocks, the agates picked up on 

 the sea-shore and elsewhere, the beautiful crystals 

 known as cairngorms, amethysts, and others, are 

 all found to be but varying forms of the same sub- 

 stance, coloured sometimes by adventitious matter, 

 as iron, &c. ; and he finds, too, that the exquisite 

 skeletons of some of the sponges, the delicate valves 

 of the diatomacese and other minute specimens of 

 organic life, consist of this very same silica, which is 

 indeed one of the most important compounds enter- 

 ing into the structure of the earth's crust. Suppose 

 the student next picks out one of the felspar 

 crystals : this on analysis will be, as was the quartz, 

 found to be also a combination ; in it he will also 

 find silica, but the silica in this instance is found to 

 be combined with the metals aluminium and 

 potassium, — in fact, is a double silicate of alumina 

 and potash. There are many varieties of felspar, 

 some of them differing from that most common in 

 granite, which is called orthoclase, in containing lime 

 or soda instead of potash; these are also distinguished 

 from the orthoclastic species by their crystalline 

 structure, which will afford, as we shall see, a ready 

 method for their recognition when they are micro- 

 scopically examined. When the granite rocks 

 become decomposed, as they often do in Cornwall 

 and elsewhere, through the wear and tear of the 

 weather, we frequently find the disintegrated mate- 

 rials so separated that the silicate of alumina of the 

 felspar forms thick deposits of the beautiful white 

 clay known as Kaolin, and which is so valuable to 

 the china-manufacturer. 



The mica of granite is usually a variety called 

 Muscovite, or potash mica ; this again on chemical 



analysis is found to contain, as did the felspar, 

 silica, alumina, and potash, and also often some iron 

 and manganese. There are several different sorts 

 of mica, also, sometimes found in granite, especially 

 Biotite, the composition of which varies from the 

 above ; but all the micas may be known by their 

 being found in flattish crystals, which may be split 

 up into an infinity of thin leaflets. Thus far our 

 unaided eyesight and the help of the chemist have 

 shown us what granite is made of; but we are now 

 beginning to learn that, would we know something 

 of the real history of a rock, a far minuter examina- 

 tion is needful, and geologists are rapidly learning 

 that they must turn to the microscope if they would 

 receive answers to many important questions, both 

 as to the history and also as to the composition of 

 rocks. A marvellous light has been shed during 

 the past few years on rock-structure through this 

 minute investigation, especially with the aid of 

 polarized light. The intricacies of the closest- 

 grained rocks have been disentangled, their com- 

 ponent parts distinguished from each other, and the 

 very order and history of their combination in the 

 mass revealed. Now, when we examine our granite 

 beneath the microscope, which can be done by 

 having thin slices prepared, we shall learn some- 

 thing about it which we could hardly hope to have 

 discovered without this aid. There has been much 

 speculation as to the origin of granite, whether it is 

 a plutonic — that is, an old volcanic rock — or whether 

 it is only a deposit from water consolidated and 

 altered during the lapse of long ages by heat and 

 pressure : the microscope will help us to the truth. 

 When magnified and examined with the polariscope, 

 a thin section of granite is a very beautiful object, 

 and its different constituent parts stand revealed 

 with the greatest distinctness : we at once learn to 

 see the crystals of felspar, somewhat opaque and 

 cloudy as they usually are in granite, but now and then 

 clear and beautifully striped, and also the crystals 

 of mica, embedded in the clear quartz, which will 

 be at once known by its bright clear colours and by 

 the margin of rainbow-like tints which border its 

 patches. Ordinary orthoclase felspar is usually some- 

 what opaque and dirty-looking under the microscope 

 aud by this it may be distinguished from the clear 

 glassy sanidine which is frequently found in igueous 

 rocks, and presents under the microscope, when 

 polarized, pure rich colours as well as sharply- 

 defined crystals similar in form to those of the com- 

 mon orthoclase. The orthoclastic felspars may be 

 very readily distinguished from the plagioclastic by 

 their structure, as revealed by the polariscope ; the 

 latter invjriably are seen to be striped with vari- 

 ously coloured bands, showing what is called twin 

 crystallization;! and the orthoclase, though often 

 form.ng twins on a larger scale, does not present 

 the minutely banded appearance of the plagioclastic 

 felspars. The mica in the granite section will not 



