170 



HARDWICKE'S SCIENCE-GOSSIP. 



[Aug. 1, 1S70. 



abundance of this mineral that I am sometimes so 

 friable or " rotten," as the felspar decomposes, and 

 then causes the other minerals to fall asunder, just 

 as the bricks of a wall would if all the cementing 

 mortar were to decompose away. In many districts, 

 as iu Cornwall, where granite comes to the surface 

 and has been subjected to atmospheric wear-and- 

 tear for hundreds of ages, it is not uncommon to 

 find the fine felspar washed into a newer deposit. 

 Such is the well-known " kaolin," or China clay of 

 commerce. The chemical composition of felspar is 

 more complex than that of quartz. For instance, 

 although its commonest elements are silica and 

 alumina, — the former the base of common sand, and 

 the latter of clay,— there are also contained in it 

 more or less of soda and potash, lime, magnesia, and 

 iron. Mica, the next commonest mineral I possess, 

 is so well known as hardly to need description. All 

 my readers are surely familiar with the small, thin, 

 silvery-looking scales contained in almost every 

 piece of granite. Its ingredients are much like 

 those of felspar, only differently mixed. Frequently 

 Hornblende is a mineral entering into our compo- 

 sition, and my listeners will readily remember it from 

 its black or dark olive-green colour. When it is 

 very abundant, it produces a rock varying from dark 

 grey to black. A great number of what may be 

 termed varieties of hornblende are known to mi- 

 neralogists. Its chemical composition, generally 

 speaking, is about one-half silica, more than a quarter 

 magnesia, and little more than half a quarter lime : 

 besides these, there are usually traces of iron, 

 alumina, and fluoric acid. 



I mentioned above that I had many relatives, who 

 were more or less nearly connected (I cannot say 

 by blood, but by mineralogical similarity of composi- 

 tion). These take various names, on account of their 

 leading peculiarities. Among these the commonest 

 is Porphyry, which takes its name from the purple 

 variety used by the ancients in making vases, &c. 

 This my hearer may know from the large and distinct 

 crystals, usually of felspar or quartz, which are im- 

 bedded in the granular matrix. Through porphyry 

 granite passes into all sorts of allied igneous rocks, 

 such as Claystone Porphyry, Clinkstone Porphyry, 

 Felspar Porphyry, and so on. When hornblende 

 takes the place of mica in the composition of granite, 

 the latter goes by the name of Syenite ; when talc 

 supplants mica, the result is called Protogine. A 

 fine-grained compound of felspar and granite, with 

 equally minute scales of mica, gives to us the varietal 

 name of Pegmatite. According to the number of 

 minerals entering into our composition, I and my 

 relatives are roughly classed as Binary, Ternary, and 

 Quaternary granites. All this detail of structure 

 may sound very dry and tedious ; but it is abso- 

 lutely necessary to go through with it, if my listeners 

 wish to be more intimate with me. 



Although I have not a distinct recollection of my 



birth (as indeed, who has?), yet I have more than a 

 suspicion that such elements as soda, potash, lime, 

 &c., greatly assisted as fluxes iu bringing me into 

 my original molten condition. I have mentioned 

 the great number of relatives which claim near or 

 distant kinship with me, and I have now only to 

 remark that their affinity to myself has been deter- 

 mined solely by the different circumstances attend- 

 ing their origin. I distinctly and utterly refute 

 the idea that the first-formed crust of the globe was 

 a granitic one ! I am fully persuaded it could not 

 possibly have been granite, and I will give you my 

 reasons by-and-by for this seemingly bold asser- 

 tion. What that cooled crust was, I doubt if 

 science will ever be able to discover. But the fact 

 that it was not granite does not in the least invali- 

 date the theory that every sun,, planet, and satellite 

 was so condensed from nebulous matter. This 

 theory must rest on other grounds, and, singularly 

 enough, additional facts are coming to its support 

 every day. We have not the slightest idea of what 

 the primitive rock or crust of the globe was. The 

 antiquated notion that it must have been granitic 

 arose out of mistaken associations. It was found 

 that, however old might be a stratified rock, whether 

 containing fossils or not, some variety or another 

 of granite was older still. Hence followed the hasty 

 deduction, that originally one granitic crust encir- 

 cled the fluid matter of the interior of the earth. 

 It was shown how subsequent rocks were them- 

 selves formed outof the wear-and-tear of this granite, 

 how the latter was in many places covered up by its 

 own debris, and how the so-called metamorphic rocks 

 were those first formed as stratified deposits, but 

 altered to their present appearance through the 

 intense heat of the newly-created seas, along whose 

 bottoms they had been elaborated ! 



All this is wrong, and it behoves me now to 

 descend from the region of pure hypothesis to that 

 of pure fact. It is just possible, speaking generally 

 of all the varieties of my family, that Protogine 

 may be oldest. This, however, has never been 

 thoroughly determined. One of my reasons for 

 believing I could not have required any very great 

 heat to reduce me to the molten condition, and 

 that in this process the agency of water, as well as 

 of heat, was necessary, is as follows : — Many of the 

 larger quartz crystals entering into my composition 

 are hollow. Frequently these hollows are more or 

 less filled with water. Now, it is a known fact 

 that molten matter at a white heat requires its 

 temperature to be considerably lowered before it 

 can even evaporate the water mechanically mixed 

 with it. It has been recently shown that crystal- 

 lized matter which has undergone pure igneous 

 fusion, has usually cavities in its crystals, not con- 

 taining water, but either stony matter or a kind cf 

 glass, and, in many cases, even a perfect vacuum. 

 Hence the conclusion is arrived at that in the case 



