62 DR. THOMAS STEER Y HUNT OK THE 



to determine what deposits of maguetite and of hematite are to be ascribed to the one and 

 what to the other origin. 



§ 123. We have seen, among the secretions of basic rocks, lime-alumina silicates, 

 like epidote and prehnite, in which the ratio of the protoxyd-bases to alumina, instead of 

 being 1 : 3, as in the feldspars and the zeolites, is IJ : 3 or even 2 : 3. Although, probably 

 on account of their solubility and their instability, we do not kuow of any natural sili- 

 cates with a still larger proportion of lime to the alumina, we have indirect evidence of 

 their former existence in solution, in the frequent occurrence of double silicates of magnesia 

 and alumina, in which the oxygen-ratio of E, al, instead of being 1 : 3, as in the feldspars, or 

 2 : 3, as in prehnite, becomes 3 : 3 and even 6 : 3, as seen in the magnesian micas and in the 

 chlorites. Such silicates, often with epidote, abound in the rocks of Huronian age. 



This process by which, through the intervention of silicated secretions from the sub- 

 stratum, the magnesian salts are removed from the sea-water, is, as we have shown, the 

 reverse of that which takes place through the action of the carbonates from the subaërial 

 decay of silicated rocks precipitating lime-salts and giving rise to magnesian waters, if not 

 over oceanic areas, at least in inland basins of greater or less extent. Alternations of this 

 kind must have been frequent in geological history, and we have evidence of a wide- 

 spread phenomenon of this kind following the Huronian age, when in seas, from which 

 magnesian salts were apparently for the most part excluded, were deposited the gneisses 

 and mica-schists of the Montalban series. These, in very many places, are found resting 

 directly, often in unconformable superposition, upon the older or Laurentian gneisses, but 

 elsewhere upon the Huronian, showing the intervention of extensive movements of eleva- 

 tion and subsidence, and probably of denudation, subsequent to the Huronian time. 



§ 124. The introduction on a limited scale, into the sea-basins of the Montalban time, of 

 magnesian salts is evident from the occasional appearance of magnesian silicates in the Mont- 

 alban rocks. The most noteworthy fact in their history is, however, the appearance in 

 this series, with gneisses which differ from those of older times in being finer grained 

 and less granitoid, of deposits containing aluminous silicates characterized by a diminished 

 proportion of protoxyd-bases. Such as these are the beds of quartzose schists holding 

 non-magnesian micas and the simple silicates, andalusite, fibrolite and cyanite. It has 

 already been mentioned that, in the formation of these rocks, the more or less completely 

 decomposed feldspar from the subaërial decay of older crenitic rocks may have been brought 

 into the areas of deposition. Either such clays, still retaining a portion of alkali from 

 undecayed feldspar, or else admixtures of kaolin with the elements of a feldspar or a 

 zeolite might, as has been suggested, yield by diageuesis, muscovite and quartz, with one 

 of the simple aluminous silicates just named. That a process of subaërial decay was in 

 progress in the Montalban time is shown by the presence in the mica-schists of this 

 series, at several localities in Saxony and elsewhere, as described by Sauer and subse- 

 quently noticed by the present writer, of " boulders of decay," having all the appear- 

 ance of those formed during the atmospheric decay of the older gneisses.''^ The 

 intervention in the deposits of that period of somewhat basic zeolitic minerals, is shown 

 by the presence in the younger gneissic series of Germany of large masses of so-called 



"" Sauer, in 1879, Zeitsclirift f. d. ges. Naturwiss. Bandlii ; also Hunt, Amer. Jour. Sci., 1883, vol. xxvi. p. 197, 

 and Ti-anB. Roy. Soc. Can. vol. i., part 4, p. 194. 



