100 



KNOWLEDGE 



[M\Y 1, 1895. 



a peir-sliaped mass by means of a silk-like thread. When 

 the estsfs are safely deposited within its interior, the mile 

 fish immediately mounts g;nard, and has been known to 

 continue uninterruptedly at his post for upwards of three 

 ■weeks. Should any dainao;e happen to the nest, so thit 

 the precious eg.'s lie open to the attack of any predaceous 

 wanderer, the janitor forthwith sets to work with the 

 greatest fnt-ryy to repair the damage, poking Iris no^e in'o 

 tiie strue'ure, and reai ranging tlie materials till all is made 

 right. Ni-sts are also made liy the freshwater species, and 

 fuirdfd With the <ame care ", the male frequenly stirring 

 up the egg-; witli his sn nit. and often Ke^piu^; up a ftu-like 

 m ivemeQt of his fins for tin appirent purpose of ensuring 

 a continual change of the water. Mmy other tislies 

 construct more or less elaborately formed nests, but even 

 wlien no ne-:t is built, the mules are ia the habit of 

 moulting guard over the eg'is ; this being the cise with the 

 bow-fla (Aiiiia calca , so abundant in the lakes of North 

 America. 



Such are some of the chief instances among amphibians 

 and fishes where special arrangements — either of structure 

 or of habit — are made for the protection of the eggs and 

 young ; and although these bear but a small proportion to 

 the cases where the latter are left to themselves, yet they 

 are sufBcient to show that in these respects these two 

 animals present peculiarities unknown among other verte- 

 brates. Why such special arrangements have been evolved 

 in these cases, or whethpr the groups in which they occur 

 have any advantage in the struggle for existence over their 

 fellows, a'-e questions which, for the present at least, must 

 remain unanswered. 



THE PLACE OF IRON IN NATURE. 



By John T. Kemp, M.A.Cantab. 



FEW elements are more abundant in nature than 

 iron, whilst none is more widely distributed. Its 

 compounds pervade every portion of the earth's 

 crust. Among massive and stratified rocks alike, 

 ferruginous deposits exist on an enormous scale, 

 frequently assuming mountainous dimensions or covering 

 mauy hundred square miles. Tlie variety of their composi- 

 tion is hardly less remarkable. Thus the useful ores 

 include ferric oxide (FejOo), known in the crystallized 

 condition as specular iron ore, and in the amorphous state 

 as hiematite ; the magnetic oxide (Fe^Oj), or magnetite; 

 ferric hydrate (FeoO™ f water), which occurs sparingly in 

 the crystalline form as the mineral gothite (Fe20.;,H30), 

 but abounds in the amorphous condition of hmonite 

 (2Fe203,3H;O, but probably a mixture of several 

 hydrates) ; titaniferous iron, a mixture of ferric oxide with 

 a variable proportion of titanic oxide (TiO.,) ; ferrous 

 carbonate (FeCOj), or spathic iron ore, with impure 

 varities known as clay ironstone. To these must be 

 added iron disulphide (FeS^h of which two crystalline 

 modifications occur, viz., iron pyrites, commonly met with 

 in the form ot brass-yellow cubes, and marcasite, much 

 lighter in colour with a radiated structure. Among less 

 abundant but noteworthy compounds may be mentioned 

 magnetic pyrites (FcgS^); copper pyrites (Cu,S,FejS3), 

 one of the most abundant ores of that metal ; mispickel, 

 or arsenical pyrites (FeSAs), the principal source of 

 arsenic ; vivianite, a ferrous phosphate of variable composi- 

 tion, met with in b. ds in wmch animal matter has decayed, 

 often of a brilliant blue colour. 



A f«w illustrations of the magnitude of sime ferruginous 

 deposits may htr^i be quoted. Pilot Knob, in Missouri, a 



hill seven hundred feet high, consists almost entirely of a 

 single mass of hfematite. Near Gellivara, in the north of 

 Sweden, a mountain of magnetite exists, whose dimensions 

 are reported as sixteen thousand feet long, eight thousand 

 feet broad, and two thousand feet high. Beds of magnetite 

 are met with among the Archiean rocks of Canada up to 

 two hundred feet in thickness. In the same region are 

 immense deposits of bse natite, titaniferous ore, and iron 

 sulphides. Zirkel describes Erzberg.a mountain in Styrii, 

 rising two thousand feet above the neighbiuring valley, as 

 composed almost exclusively of spathic iron ore. 



Besides those ferruginous deposits which from their 

 form or dimensions are entitled to rank as independent 

 rock masses, hosts of siniller aggregations are met with, 

 such as veins, encrusting layers, nodules, and scattered 

 crystals. Thus haematite often occurs in veins traversing 

 crystalline rocks, whilst layers of ferric hydrate are 

 deposited in their channels by waters containing iron, both 

 above and below the surface. Many of the septarian 

 misses so common in clayey strata consist essentially of 

 clay ironstone. Hiematite nodules, often containing fossil 

 remains, abound among some of the carboniferous beds. 

 Masses and single crystals of iron pyrites occur plentifully 

 in some strata, marcasite in others, but what conditions 

 determine the form assumed by the sulphide we do not 

 know. The various " greensands " owe their appellation 

 to the presence of grains of an iron silicate of very variable 

 composition, known as glauconite ; deposits of the same 

 mineral are now forming in certain parts of the sea-bed. 

 Magnetite may here be mentioned as an essential con- 

 stituent of basalt and other volcanic rocks, in which it 

 occurs in the form of opique octahedral crystals. 



The most striking evidence of the universal presence of 

 iron in nature is, however, found in the colours imparted 

 by its compounds. Iron has ju-tly been called " the 

 great pigment of nature." Few deposits there are which 

 are not tinged with iron in one chemical form or another. 

 To it are due the brown, yellow, red, green, blue and 

 creamy tints which in endless variety characterize the 

 vast majority of rocks. Green and blue colorations are 

 produced generally by ferrous compounds, red by ferric 

 anhydride, and yellovv and brown lints by ferric hydrates. 

 The presence of other substances, such as carbonaceous 

 matter, largely affects the coloration in many instances. 



Probably not more than eigut or possibly ten of the 

 elements occur in the earth's crust in larger proportion 

 than iron. The significance of this fact will be appreciated 

 when it is added that ninety-nine out of a hundred parts 

 by weight of the crust are estimated to be composed of 

 some sixteen elements at the most, leaving fifty or more 

 which constitute the remaining one-hundreth part. Never- 

 theless, in comparison with oxygen, silicon and aluminium, 

 of which about eighty-five per cent, of the accessible rocks 

 consist, a decidedly low place must be assigned to iron as 

 constituting probably less than one per cent, of the whole, 

 so rapidly does the relative abundance of the elements 

 fall off. About half of the earth's crust is composed of 

 oxygen. 



Iron is, as would naturally be expected from the 

 universality of its occurrence elsewhere, one of the 

 elements, some thirty in all, which have been detected in 

 the oceanic waters. Messrs. Thorpe and Morton report 

 the presence of ferrous carbonate to the extent of one 

 part in two hundred thousand in the water of the Irish Sea 

 collected during winter. This proportion, if maintained 

 throughout the ocean, would indicate the existence of 

 more than four billion tons of metallic iron in solution. 



In the organic world, again, iron appears to play an 

 indispensable part. It is an essential constituent of the 



