04 



THE GEOLOGIST. 



cn'stalline rocks, and to tlie researches of Daubree, Senarmoiit, and others, the 

 author remarked, in the first place, that tlie problem of the generation, from the 

 sands, clays, and eartlw carbonates of sedimentary deposits, of the yarious siliceous 

 minerals which make up the crystiilhne rocks, may be now regarded as solyed ; and 

 that we find the agent of the process to be water, holding in solution alkaline car- 

 bonates and silicates, acting upon the heated strata. Under some circumstances, 

 howeyer — such as the presence of gyj^sum or magnesia — such anomalies might 

 occur as are presented by the comparatiyely unaltered condition of some portions 

 of the strata in metamorphic regions. 



§11. Many crystalline rocks, formerly regarded as of plutonic origin, are now 

 found to be represented among altered "sedi\nentaiy strata ; and the chemical stu- 

 dent in geology is now brought to the conchision that metamorphic rocks, such as 

 gi-anite, diorite, dolomite, serpentine, and limestone, may, under certain con- 

 ditions, appear as intrnsi\ e nxks. This is cliiefly owing to the pasty or semi-fluid 

 state which those rocks nnist haA C assumed at the time of their displacement. 



§ III. Tlie author next remarked that the in'omulgated h}-potheses relating to 

 the origin of the two great groups of plutonic rocks— those with potash and much 

 silica, and those with soda and less silica — are not satisfactory. 



§ ly. My. Hunt, considering that the water of the early palaeozoic ocean diflered 

 from that of the modern seas, in that it contained chlorides of calcium and mag- 

 nesium to a for greater extent, especially the former, sulphates being present only 

 in small amount, noticed that the replacement of the cliloride of calcium by common 

 salt inyohwi the intervention of carbonate of soda and the formation of carbonate 

 of lime : ;ina that the continual decomposition of alkaliferous silicates to form the 

 yast masses of argillaceotts seilinients from the felspathic minerals of the earth's 

 crust, must haye formed, aud is still forming, alkahne cai-bonates which play a 

 most important part in the chemistry of the seas. 



§ y. The study of the chemistry of mineral waters, in connexion with that of 

 sedimentary rocks, leads the author to beheye that the result of processes continu- 

 ally going on in nature is to di\ide the silico-argillaeeous rocks into two gTeat 

 classes : the one characterized by an excess of silica, by the predominance of 

 potash, and by the small amounts of lime, magnesia, and soda, and represented 

 by the granites and trach3i:-es ; while in the other class silica and potash are less 

 abundant, and soda, lime, and magnesia prevail, giAing rise (by metaiuorphism) to 

 tricliiiic felspars and p>Toxenes. "The metamorphism and displacement of sedi- 

 ments may thus, he observed, enable us to explam the origin of the different 

 yiii'ieties of plutonic rocks, without calling to om- aid the ejecttons of a centi'al fii*e. 

 (See § III.) 



§ yi. The most ancient sediments, like those of modem times, were doubtless 

 composed of sands, clays, and limestones ; but, on the principles laid down in 

 §§ ly. and y., the author shows that the chemical composition of the sediments 

 in different geologic periods must liave been ia'adually changmg. Illustrating his 

 yiews by the condition of the Canadian rocks, ]Mr. Hunt observes that, on the 

 large scale, in the more recent crystalline or metamorphic rocks, we find a less 

 extensive development of soda-felspar, while orthoclase and mica, chlorite and 

 epidote, and silicates of al\nnina. like chiast elite, kyanite, and staurotide (which 

 contain but little or no alkali, and are rare in the older rocks\ become abundant. 

 The decomposition, too, of the rocks is more slow now, because soda-silicates are 

 less abundant, and because the proportion of cai'bonic acid in the air (an etficient 

 agent in these changes) has been dimmished by the formation of limestones 

 and coal. 



§ y 11. The author accepts the views of Babbage and Herschel as to the internal 

 heat of the earth rising through the stratified deposits, on account of the supeiticial 

 accumulation of sediments, metamoiithosing the rocks submitted to its action, 

 causing earthquakes and volcanic irruptions by the evolution of o-ases and vapours 

 from chemic^il reactions, and giN"ing rise to disturbances of equilibrium over wide 

 areas of elevation and subsidence. " 



§ yill. ]\[r. Hunt observes that the structure of moimtain-chaius, both those 

 due to the uprise of metamorpliosed rocks through tertiary and secondary deposits, 



