Februaky 1, 1898.] 



KNOWLEDGE. 



P^ 



IlLUSTRATED MAGAZINE 



&NCEJL1TERAT 



Founded in 1881 by RICHARD A. PROCTOR. 



LONDON : FEBRVAEY 1, 1S98. 



CONTENTS. 



The Floor of a Continent. By Geenvii.le A. .T. Cole, 



M.R.I. A., F.a.s. {Illustrated) 



Economic Botany. By John E. Jacksox, a.l.s., etc. 

 From a Hole in the Mudflats. By Habrt F. Withbrby, 



F.Z.S., M.B.O.U. (Iltustraled)... 

 Liquid Fluorine. By C. F. Townsejjd, F.c.s. (Illustrated) 

 Letters :—L. Paxton; "G. E. E."; Feed. TVniTTERON; 



Joseph P. Nttitn; J. Ernest Gbubb; W. H. Cock; 



H. U. Jeffert; Ivo F. H. CarrGeegg... 



British Ornithological Notes. Conducted by Harry F. 



WiTHERBY, F.z.s., M B.o.r. (Illustrated) 



Science Notes 



Notices of Books 



Short Xotices 



Books Received 



Total Solar Eclipse, January 22nd, 1898. 



Photograph of the Spiral Nebula Messier 33 Trian- 



guli. By Isaac Roberts, d.sc, f.e.s. (Plate) 

 Moon in Eclipse. January 7th. 18 By L. Paxton ... 



The Spectra of Bright Stars. By E. W. M.\under, 



F.R.A.S. ... 



Ancient Red Deer Antlers. By R. Ltdekkee, b.a., p.e.s. 



{Illi'sfrafed} 



Notes on Comets and Meteors. By W. F. Denning, 



F.R.A.S. ... ... ... ... ... ... 



The Face of the Sky for February. By Heebert 



Sadler, f.e.a.s. ... 



Chess Column. By C. D. Locock, b.a 



THE FLOOR OF A CONTINENT. 



By Grenville A. J. Cole, m.r.i.a., f.g.s., Professor of 

 Geolo'ifi in the Royal ( 'ollege of Science for Ireland. 



WHEN we consider the thickness of the sedi- 

 mentary deposits that lie beneath us at any 

 point on the surface of the earth, and 

 compare them with the depth of four 

 thousand miles that separates us from the 

 earth's centre, we may come to regard the whole stratified 

 series as a mere blanket on the true substance of the globe. 

 Eversincethecrust became solid — ever since theatmosphere 

 cooled and the rain began to fall — the earth's surface has 

 been subject to denudation, and the dust and mud of ii 

 have been carried into the shallow depressions that have 

 formed in it from time to time. Wrinklings of the crust 

 have uplifted these layers of earth-dust, and have folded 

 them, together with more fundamental matter, into 

 mountains and continental margins. In the sections thus 

 revealed, the sweepings of the earth— the sedimentary 



series — assume to our eyes magnificent proportions ; but 

 every now and then we have a glimpse of the real body of 

 the earth (or, rather, of its real skin), cleaned from this 

 dust of ages. In no spot on the globe have all the strati- 

 fied rocks that are known to us been piled continuously 

 one upon another ; but, even if this had been the case, they 

 would have formed a layer less than twenty-five miles thick. 

 If we represent the earth's radius by ten inches, this layer 

 would appear, on the same scale, as less than one-sixteenth 

 of an inch. 



Where, indeed, denudation has long been active, as in 

 the northern regions of Europe and America, we find 

 ourselves in the presence of a vast bared surface, in which 

 there is little to remind us of the sediments of ordinary 

 geological periods. Here and there, isolated relics, like 

 the marine .Jurassic beds of the island of Ando, suggest 

 to us the coating of stratified rocks that once spread over 

 much of this denuded area ; but the main masses are of 

 Pre-Cambrian age — that is, they underlie the beds that 

 contain the oldest clearly recorded fauna on the globe. 

 Here, then, we seem to be in touch with the true substance 

 of the crust — with the floor on which our filmy continental 

 or oceanic accumulations rest. 



Without entering into microscopic details, we may see 

 that there is a remarkable uniformity of character in the 

 rocks that form this floor. Gneisses, resembling granites, 

 but with a " streaked out " and even banded arrangement 

 of their constituents, form the largest portion of the 

 mass. Their chemical composition* almost always shows 

 a high percentage of silica, and the alkalies amount to 

 five or even eight per cent. Their essential structure, the 

 " foliated " arrangement of their mineral constituents, 

 may have been induced in them by pressure after they had 

 become practically solid, or by the flow of the whole mass 

 while the crystals were still in course of construction. 

 The larger constituents thus possess a lenticular form, as 

 if drawn out at their edges ; and these lenses lie in similar 

 positions throughout considerable masses of the rock. 

 The smaller constituents seem to have flowed round 

 about them, streaming on in fairly parallel layers ; and 

 thus "foliation-planes" have been set up, along which 

 even coarse-grained gneisses tend to split when struck. 



In many gneisses there are distinct rock-bands, some 

 bands, for instance, resembling mica-schist, while others 

 resemble fine-grained granite, rich in quartz and felspar 

 (Fig. 1). In such cases it is quite possible that one type 

 of rock has intruded into another in fine parallel sheets,! 

 or that a viscid mass of varied composition has been pressed 

 out underground, and so has received a gneissic structure.! 

 Sometimes above the typical gneisses, and sometimes 

 associated with them, there is usually a series of crystalline 

 rocks of much finer grain and of greater variety of com- 

 position. Foliation is present in them, and they are 

 classed collectively as schists. Mica-schist, a foliated 

 mixture of quartz and mica (usually muscovite), and 

 commonly accompanied by red-brown garnet, is the type 

 most extensively developed. The schists present many 

 analogies with sedimentary rocks, and many mica-schists 

 have undoubtedly arisen from the extreme alteration 

 of sediments under heat and pressure ; but the planes of 

 foliation only rarely correspond to those of original depo- 

 sition, and the crystalline character of the constituents 



* See, for instance. Roth, " AUgemeine imd chain. Geolo^ie," Bd, 

 II., p. 397. 



t Compare A. C. Lawson, " A Multiple Diabase Dvke," American 

 Geologist, Vol. XXVI., p. 29(5. 



X See Sir A. Geilde and J. J. Teall, " On Banded Structure of 

 Gabbros in Skye," Quart. Journ. Geol. Soc, Vol. I>., p. 6.57, and Plate 

 XXVI. 



