316 W. 31. Hutchings — The Origin of some Slates. 



VIII.— Notes on the Probable Origin of Some Slates. 



By "W. Maynard Hxtchings. 



(Concluded from page 273.) 



IN the deposits previously described what takes place appears to 

 be about as follows. The larger flakes of biotite undergo an 

 alteration which largely produces epidote, and not improbably 

 granular rutile, though this is not at all proved. Crystals of rutile 

 very rarely result during the process. Such of the iron of the biotite 

 as is not combined in the epidote (which is strongly coloured) is 

 removed in solution, some of it being deposited again as limonite 

 diffused throughout the deposit, and some taking part in the forma- 

 tion of the thin bands of ironstone, being precipitated as carbonate. 



Saline solutions dissolve and carry away part of the silica ; finally 

 the whole of the constituents of the mineral are broken up, partly 

 removed in solution, or diffused among the other materials of the 

 deposit. Of the titanic acid contained much is probably removed in 

 solution jointly with the silica, and this dissolved titanic acid may 

 most likely, as Thurach suggests is the case in some sandstones and 

 other sedimentary rocks, supply by re-precipitation (by means of 

 carbonic acid or otherwise) and crystallization, the material for the 

 secondary crystals of anatase. It may also, diffused through the 

 mass, take part with the titanic acid of the smaller fragments of 

 biotite in yielding the countless small crystals of rutile : and we 

 may very reasonably ascribe to it partly also the formation of the 

 thin plates of micaceous ilmenite. 



The flakes of muscovite of anything like noticeable size do not 

 appear to undergo any sort of change, so far as these particular 

 deposits are concerned. 



The small, and very small, flakes and shreds of biotite appear to 

 undergo an alteration which differs in many ways from that suffered 

 by the larger bits, and which seems to depend not merely on 

 ordinary attack by water and by saline solutions, but to be also, and 

 more largely, brought about by chemical inter-action between the 

 biotite and other minute materials with which it is deposited in 

 intimate contact. This fine mixture of biotite, muscovite, kaoline, 

 the minutest waste of felspar, 1 and in less degree of quartz, and 

 probably other substances, under the joint action of pressure, warmth, 

 and mineral solutions, gives rise to various decompositions and re- 



1 The experiments of Daubree (Geologie experimentale, 1879) on the effects of 

 the mutual attrition of fragments of granite in water, are of very great interest and 

 bear strongly on the special point here in question. It is to be particularly noted 

 that he shows how comparatively .rapidly the felspar is reduced to the condition of 

 the finest mud ("limon"); and further, that the felspathic mud so produced has 

 not been simply reduced mechanically to powder, but has been chemically acted upon 

 to a noticeable extent in a relatively short time. The felspar has given up some of 

 its alkali to the water and has become hydrated by taking water into combination. 

 The granite used in these experiments was fresh and its felspar not weathered. 



This fine mud of hydrated felspar, so fine as to remain suspended in water several 

 days, and depositing as a plastic mass, is in just the condition for rapidly undergoing 

 chemical changes and acting powerfully upon the minute fragments of biotite, etc., 

 with which it is intimately intermingled. 



