Feb. 28, 1884] 



NA TURE 



419 



AGATES 

 'PHE following letter was addressed by the writer in 1871 to 

 Mr. Joseph John Murphy, and though not originally 

 intended for publication, is now published with the writer's 

 consent ; — 



St. Andreius, November ^, 1871 



Dear Sir, — I have on my return found your note as to 

 agates. Though I have been at work on the subject in different 

 ways for many years, I have not found myself in a position yet 

 to publish. In fact I cannot yet say that I know much as to how 

 they have been formed, though I do know, or rather am able to 

 show, that they have not been made in the manner usually 

 supposed. 



The late Principal Forbes conceived that they had been 

 formed by concentric deposition round a central nucleus : — this I 

 showed him to be untenable. Others conceive that siliceous 

 matter in a state of fusion has been poured into cavities throuvfh 

 an opening, such opening being called the "point of infiltra- 

 tion." I am able to show that this so called point of infiltration 

 is an orifice of escape or exit of something. 



Fully to state how (from examination of their mode of occur- 

 rence, experiments upon the decomposibility of trap rocks under 

 the action of carbonated w'ater, section of agates in every con- 

 ceivable direction, experiments upon their powers of absorbing 

 liquids, and from microscopic examination) I conceive agates 

 to be formed would call for indeed a long statement. 



I will attempt briefly to put it thus : — 



Igneous rocks are being pom-ed forth from a volcanic vent, in 

 perfectly fluid or at least plastic flow ; some are dense, some 

 scoriaceous, some frothing, and so when solidified are vesicular, 

 or perchance even hold in suspension bubbles of included water, 

 this latter holding in solution (red-hot solution) solids afterwards 

 to separate as rheolites. Should the air-bubbles of the vesicular 

 rocks arise through the plastic mass while it is motionless, these 

 bubbles will be more or less rounded or pear-shaped. Should 

 the solidifying rock, however, become crystalline or porphyritic, 

 as generally is the ca^e with amygdaloids, the separating crystals 

 of labradorite, &c., will more or less roughen the sides, and so 

 destroy the smooth and rounded figure of the cavity ; while, if 

 the lava-flow continues its motion while the bubbles are still 

 rising, their shape will be more or less flattened or altered : — try 

 bubbles in flowing treacle. 



Stage the first. — An empty cavity of any shape. 



Stage the second. — The rock, while solidifying, may contain 

 an excess of a magnesian mineral, which is exuded into the cavity ; 

 or this excess of magnesian compound (magnesia not being, to 

 any large extent, a natural constituent of the mass of a trap) 

 may be held as vapour in the cavity, to be, on cooling, deposited 

 on its sides. This forms in Scotland, Faroe, Iceland, &c., the 

 layer of celadonite or delessite ; at Giant's Causeway, of chloro- 

 phoeite, which, on the extraction of the afterwards fiUed-up 

 cavity, forms the "skin of the pebble." 



Stage the third. — One of two processes, the first very doubtful. 



The cooling and shrinking rock holds in a state of liquidity, 

 from heal, an excess of colloidal silica which is exuded into the 

 cavity forming a chalcedonic druse. But, admitting the process, 

 it must here stop, and a solid agate could not thus be formed. 

 This seems to have been the view of Sir George Mackenzie. 



Tlie other process I pin my fiith to. The thoroughly solidi- 

 fied — indeed the now old — rock is having its felspar (labradorite 

 or otlier) decomposed by water holding carbo.iic acid in solu- 

 tion. I have proved that this process is rapid and c mtinuous, 

 and agate-holding trap; are all rotten ; the colloidal silica, with 

 a certain quantity of tridamyte is taken up by this water, and 

 transfuses into the cavity; the silica is there solidified — probably 

 the layer of delessite is the coagulation. We have now a cavity 

 .slightly lined with chalcedonic matter, containing, within, water 

 more or less pure, while without (that is outside of the now 

 double skin, delessite and first layer) we have a strong solution 

 of colloidal silica constantly supplied. Endosmose and exos- 

 mose are set up with all their resistless force. The strong solu- 

 tion finds its way through the two or any number of increasing 

 skins : the w-.ak water is forced out through the " point of 

 infiltration," and so in its passage out thins all the successively 

 depo-ited layers (7? rta//>/a(re'. By this continuous flow of col- 

 loidal silica (held in solution by liquid) through the already 

 coagulated or deposited layers, continuous coagulation of the 

 silica in the yet hollow agate, and continuous extrusion of the 

 residual water, we have the ultimate filling up of the cavity, and 

 a solid agate formed. 



The adhesion of agates to the containing rock is slight in 

 most ca^es from the so-called "skin" being magnesian and 

 soapy. 



The " point of infiltration," instead of being at once filled up, 

 as would result from the inflow of coagulable silica, is in reality 

 the last point filled up, being truly the point of escape : indeed 

 it frequently is not altogether filled up, remaining an open tube. 



The microscope shows on a cross section the concentric layers 

 of coagulated silica, soluble in alkalies ; the crystals or fibres of 

 tridamyte cross these layers at right anijles, radiating like a 

 rheolite from the skin, and it is always along the sides of these 

 crystals that intruding and staining liquids find a way ; probably, 

 therefore, along their sides also did the ingress of chalcedonic 

 fluid find entrance. I remain very truly yours, 



M. FORSTER HeDDLE 



THE ORIGIN OF THE SCENER V OF THE 

 BRITISH I SLA NDS ' 



"T^HE Lakes of Britain present us with some of the most inter- 

 esting ]5roblems in our topography. It is obvious that the 

 existence of abundant lakes in the more northern and more rocky 

 parts of the country points to the operation of some cause which, 

 in producing them, acted independently of and even in some mea- 

 sure antagonistically to the present system of superficial erosion. 

 It is likewise evident that as the lakes are everywhere being 

 rapidly filled up by the daily action of wind, vegetation, rain, 

 and streamlets, they must be of geologically recent origin, and 

 that the lake-forming process, whatever it was, must have attained 

 a remarkable maximum of activity at a comparatively recent geo- 

 logical epoch. Hardly any satisfactory trace is to be found of 

 lakes older than the present series ; perhaps Lough Neagh, which 

 from its thick deposits and their fossils, has been referred back 

 to Pliocene times, is the solitary exception. How then have our 

 lakes arisen ? Several processes have been concerned in their 

 formation. Some have resulted from the solution of rock-salt or 

 of calcareous rocks and a consequent depression of the surface. 

 The "meres" of Cheshire, and many tarns or pools in limestone 

 districts, are examples of this mode of ori-^in. Others are a 

 consequence of the irregular deposit of superficial accu- 

 mulations. Thus, landslips have occasionally intercepted the 

 drainage and formed lakes. Storm-beaches, thrown up by the 

 waves along the sea-margin, have now and then ponded back the 

 waters of an inland valley or recess. The various glacial deposits — ■ 

 boulder-clays, sands, gravels, and moraines — have been thrown 

 down so confusedly on the surface tliat vast numbers of hollows 

 have thereby been left which, on the exposure of the land to 

 rain, at once became lakes. This has undoubtedly been the 

 origin of a large proportion of the lakes in the lowlands of the 

 north of England, Scotland, and Ireland, though they are 

 rapidly being converted by natural causes into bogs and meadow- 

 land. Underground movements may have originated certain of 

 our lakes, or at least may have fixed the direction in which they 

 have otherwise been produced. A very large number of British 

 lakes lie in basins of hard rock, and have been formed by the 

 erosion and removal of the solid materials that once filled their 

 sites. The only agent known to us by which such erosion could 

 be effected is land-ice. It is a significmt fact that our rock-basin 

 lakes occur in districts which can be demonstrated to have been 

 intensely glaciated. The Ice ^ge was a recent geological episode, 

 and this so far confirms the conclusion already enforced, that the 

 cause which jiroduced the lakes must have been in operation 

 recently, and has now ceased. We must bear in mind, however, 

 tliat it is probably not necessary to suppose that land-ice excavated 

 our deepest lake-basins out of solid rock. A terrestrial surface 

 of crystalline rock. Ions exposed to the atmosphere, or covered 

 with vegetation and humus, may be so deeply corroded as, for 

 two or three hundred feet downward, to be converted into mere 

 loose detritus, through which the harder undecomposed veins and 

 ribs still run. Such is the case in Brazil, and such may have 

 been also the case in some glaciated regions before the glaciers 

 settled down upon them. This superficial corrosion, as shown 

 by Pnmpelly, may have been very unequal, so that when the 

 decomposed material was removed, numerous hollows would be 

 revealed. The ice may thus have had much of its work already 

 done for it, a id would be mainly employed in clearing out the 



* Abstract of fourtli lecture eivenattlie Royal Institution. February 26, by 

 Archibald Geikie, F.R.S., Director-General of the Geological Survey ot 

 the United Kingdom. Continued from p. 397. 



