368 D, Forbes — On the Study of Chemical Geology, 



In aqueous action, on the other hand, the water acts as a liquid, 

 not as a vapour, and is the main agency. Yet the effects of the 

 gaseous and solid constituents, as well as of its temperature, must 

 be taken into full consideration. 



The immense volumes of steam emitted by volcanoes during their 

 outbursts, would naturally prepare the observer to expect that some 

 portion might become entangled in the lava, and thus account for the 

 microscopic cavities containing water frequently foimd in volcanic 

 products ; whilst at the same time he would not consider the presence 

 of microscopic water cavities in the older rocks as proving any dis- 

 similarity of origin, or as necessarily demonstrating them to be of 

 aqueous formation, as has been advanced by some writers on the 

 subject. 



As an illustration of the fact, that the same phenomenon may 

 at times be the result of totally different agencies, Mr. Forbes 

 takes, for example, the most widely spread of all substances, 

 silica, and shows that it can be produced in the laboratory by many 

 totally distinct processes : as an igneous product by the oxidation of 

 silicon at high temperatures ; as an aqueous product by the decom- 

 position of silicates ; as a gasolitic product from the decomposition 

 of the gaseous compounds of silicon with fluorine, chlorine, etc. ; 

 and it even might be regarded as an organic product, when produced 

 from the decomposition of the silicic ethers. 



In the field the chemical geologist meets with abundant cases of 

 crystallised silica or quartz, as an igneous product occurring in the 

 lavas from volcanoes ; as an aqueous product crystallised from solu- 

 tion, or proceeding from the decomposition of mineral silicates ; as a 

 gasolitic product in the form of tubes, evidently resulting from the 

 decomposition of its fluorine compounds ; whilst the carapaces and 

 other parts of infusoria, etc., present silica in a form which owes its 

 appearance to the action of organic life. 



Mr. Forbes concludes that it is .impossible to be over-cautious in 

 attributing the formation of minerals, or of the rock-masses in which 

 they occur, to any one cause, to the exclusion of other agencies. 



He roughly arranges all rocks which are as yet known, under 

 two heads — eruptive and sedimentary ; both of which classes of 

 rocks can be subdivided respectively into normal and metamorphic, 

 i.e. those which still are found comparatively ^ imchanged, and those 

 which have suffered metamorphism or alteration, brought about by 

 either mechanical or chemical force, or by both combined. 



The sedimentary strata, when comparatively unaltered, show them- 

 selves as tuffs, ashes, breccias, conglomerates, grits, sandstones, shales, 

 clays, marls, limestones, etc., and have been all formed either by the 

 direct destruction of eruptive rocks or of previous sedimentary beds 

 which, in their turn, had so originated ; even the lime which organic 

 life has eliminated from the ocean to form the limestones, came, if 

 not altogether, at least in greater part, from the same source. This 

 has been the case even from the very oldest period, or, in other 

 words, from the epoch of the consolidation of our sphere, since the 

 * Everything in Nature appears, faster or slower, to become more or less altered. 



