94 
THE GEOLOGIST. 
crystalline rocks, and to the researches of Daiibree, Senarmont, and others, the 
author remarked, in the first i)lace, that the problem of the generation, from the 
sands, clays, and earthy carbonates of sedimentary deposits, of the various siUceous 
minerals which make up the crystalline rocks, may be now regarded as solved ; and 
that we find the agent of the process to be water, holding in solution alkaline car- 
bonates and siliavtes, acting upon the heated strata. Under some circumstances, 
however — such as the presence of gyi)suni or magnesia — such anomalies might 
occur as are presented by the comparatively unaltered condition of some portions 
of the strata in metanioiijhic regions. 
§ II. Many crystalline rocks, formerly regarded as of plutonic origin, are now 
found to be represented among altered sedimentary strata ; and the chemical stu- 
dent in geology is now brought to the conclusion that metamorphic rocks, such as 
granite, diorite, dolomite, serpentine, and limestone, may, under certain con- 
ditions, ajipear as intrusive rocks. This is chiefly owing to the pasty or semi-fluid 
state which these rocks must have assumed at the time of their displacement. 
§ III. The author next remarked that the promulgated hy))otheses relating to 
the origin of the two gTeat groups of plutonic rocks — those with potash and much 
silica, and those with soda and less silica — are not satisfactory. 
§ IV. Mr. Hunt, considering that the water of the early palajozoic ocean difl'ered 
from that of the modern seas, m that it contained chlorides of calcium and mag- 
nesium to a far greater extent, especially the former, sulphates being present only 
in small amount, noticed that the replacement of the cliloride of calcium by common 
salt involved the intervention of carbonate of soda and the formation of carbonate 
of lime ; and that the continual decomposition of alkaliferous silicates to form the 
vast masses of argillaceous sediments from the felspathic minerals of the earth's 
crust, must have formed, and is stiU forming, alkaline carbonates which play a 
most important part in the chemistry of the seas. 
§ V. The study of the chemistry of mineral waters, in connexion with that of 
sedimentary rocks, leads the author to believe that the result of processes continu- 
ally going on in nature is to divide the silico-argillaceous rocks into two great 
classes ; the one characterized by an excess of silica, by the predominance of 
potash, and by the small amounts of lime, magnesia, andf soda, and represented 
by the gi'anites and trachytes ; while in the other class silica and potash are less 
abundant, and soda, lime, and magnesia prevad, giving lise (by metamorphism) to 
triclinic felspars and pyroxenes. The metamorijhism and displacement of sedi- 
ments may thus, he observed, enable us to explain the origin of the difiierent 
varieties oi' plutonic rocks, without callmg to our aid the ejections of a central fire. 
(See S III.) 
§ VI. The most ancient sediments, like those of modem times, were doubtless 
composed of sands, clays, and limestones ; but, on the principles laid down in 
§§ IV. and v., the author shows that the chemical composition of the sediments 
in diff"erent geologic periods must have been gradually changing. Illustrating his 
views by the conchtion 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 alumina, like cliiastohte, 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 abimdant, and because the proportion of carbonic acid in the air (an efficient 
agent m these changes) has been diminished by the formation of limestones 
and coal. 
S VII. 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 supeificial 
accumulation of sediments, metamorjihosing the rocks submitted to its action, 
causmg earthquakes and volcanic irmptions by the evolution of gases and vapours 
from chemical reactions, and giving rise to disturbances of equilibrium over wide 
areas of elevation and subsidence. 
§ VIII. Mr. Hunt observes that the structiu-e of mountain-chains, both those 
due to the uprise of metamorphosed rocks thi'ough tertiary and secondary deposits, 
