60 DR. THOMAS STEEEY HUNT ON THE 



readily decomposed by the carbonic acid everywhere present, with separation of free silica 

 and carbonate of lime. From this wovxld be formed the first deposits of limestone, which 

 make their apjiearance in the old gneissic rocks and become mingled with magnesian car- 

 bonate and silicates from the introduction of magnesian salts into the waters. The 

 comparative instability of the lime-silicate is seen when wollastonite is compared with 

 the corresponding silicates, pyroxene and enstatite. It is possible, notwithstanding the 

 absence of magnesian species from zeolitic secretions, that, under certain conditions, small 

 portions of magnesian silicate may have been included in the early crenitic deposits, but 

 the rarity of such magnesian silicates in these, and their abundance in parts of the later 

 Laurentian and in younger deposits, points to a new source of the magnesian element ; 

 namely, the extravasation of portions of the underlying primary mass, and its sub- 

 aerial decay. 



It would be instructive to consider in this relation the gradual removal of a large pro- 

 portion of silica from the primary stratum in the forms of orthoclase, albite and quartz, 

 and the consequent partial exhaustion of portions of this underlying mass, so that its suc- 

 ceeding secretions consisted chielly of less silicic silicates, such as labradorite and andésite, 

 without quartz, as in the Norian series. 



§ 120. The conditions of this first exoplutonic action cannot be fully understood until 

 we haA^e settled the question of the permanence of continental and oceanic areas, and the 

 extent of the early crenitic rocks which constitute the fundamental granites and the granit- 

 oid gneisses. Whether these are spread, with their vast thickness, alike underneath the 

 great areas of the paleozoic series and our modern oceanic basins ; in brief, whether or not 

 they are universal, as supposed by Werner, is a question which cannot here be discussed. 

 There is, however, nothing incompatible with what we know of the chemistry of the 

 early rocks and the early ocean in the supposition that they were universal, since there is 

 apparently no evidence that the products of subaerial decay of exposed rocks intervened in 

 their production. Such a condition of things was, however, necessarily self-limited ; the 

 great diminution of the primary mass, from the constant removal of portions of it in a state 

 of solution, and the vast accumulated weight of the superincumbent accumulated granitic 

 and gneissic material, could not fail to result in widely spread and repeated corrugations 

 and foldings of the overlying mass, the effects of which are seen in the universally wrinkled 

 and frequently vertical attitude of the oldest gneissic rocks. Such a process, like the simi- 

 lar though less considerable movements in later times, would probably be attended with 

 outflows, in the form of fissure-eruptions, of the underlying basic stratiim, which, in 

 accordance with our hypothesis, was permeated with water imder conditions of tempera- 

 tures and pressure that must have given to it a partial liquidity. Su.ch a process of collapse 

 and corrugation of the crenitic deposits, attended with extravasation of the underlying 

 primary stratum, would, doubtless, be often repeated in these early periods, resulting in 

 frequent stratigraphical discordances, which are, however, in all cases to be looked upon as 

 local accidents, and not as wide-spread catastrophes. Hence the appearance, from time to 

 time, of exoplutonic rocks, with upliftings and depressions of the older rocks, which caused 

 the exposure of both alike to the action of the atmosphere. 



§ 121. The consequent subaerial decay of these two types henceforth introduced new 

 factors into the rock-forming processes of the time, and made the beginning of what Werner 

 called the Transition period. The decomposition of these, under the influence of a moist 



