August 2 2, 1878] 



NATURE 



443 



The suggestions I have ventured to make could not, of course, 

 be well carried out, unless the government take into its own 

 hands the appointment to all scientific chairs. Of this I think 

 I see indications. I believe that sooner or later the government 

 will assume the supreme direction of education in this country. 

 It has already taken primary education under its control, and 

 quite recently, here in Ireland, intermediate education to a great 

 extent. And does the appoiatment of so many university com- 

 missioas not show a disposition on the part of the government 

 to assume the direction of higher education also ? 



Section C— Geology. 



7%^ Origin and the Succession of the Crystalline Rocks, by Prof. 

 J. Sterry Ilunt, LL.D., F.R.S. — As a preliminary to a statement 

 of the results of many years of study of the crystalline rocks 

 in North America, the author proceeded to consider the question 

 of their origin, which is still a subject of debate between plu- 

 tonists and neptuni.-ts. The crystalline silicate rocks naturally 

 divide themselves into three groups, namely, those indigenous 

 stratified formations which have been called primitive or primary, 

 those masses to which, from their relations to conti^oas rocks, 

 geologists assign an exotic origin, and in accordance with a 

 generally-accepted theory, have agreed to call igneous or plu- 

 tonic ; and a third and di-tinct group of rock-masses which, 

 though like the la>t, clearly posterior to those encasing them, are 

 now, by most geologists, admitted to be of aqueous origin. 

 This third group includes metalliferous lodes and various other 

 crystalline veinstones, and is conveniently designated endoge- 

 nous. It is not always easy to distingiiish between the rocks of 

 these three groups ; there are not wanting those who have 

 assigned an igneous origin to metalliferous lodes, and many still 

 confound endogenous granitic veins with the mineralogically 

 similar plutoaic granites. In like manner the distinction 

 between the latter and the stratified granitoid gneisses is fre- 

 quently not very apparent. That the movement of flow in 

 extravasated plutonic rocks may give to their constituent mine- 

 rals a stratiform arrangement, is a fact of which both exotic 

 granites and doleritic dykes and masses afford illustrations. 

 Moreover, the arrangement due to successive depositions upon 

 the walls of a fissure may give to an endogenous mass a struc- 

 ture which simulates that of a sedimentary rock, and imparting 

 to granitic veinstones a resemblance to gneiss ; while a laminated 

 structure sometimes results from the arrangement of the crystals 

 developed in a cooling mass. Hence there are not wanting 

 those who include under the head of plutonic rocks not only the 

 clsarly marked exotic granites, dolerites, and diorites, but the 

 granitoid gneisses, the massive bedded greenstones, and like- 

 wise the more schistose rocks with which these gneisses and 

 greenstones are often so intimately associated that it 'is difficult 

 to separate them. According to those who hold this plutonic 

 view, the crystalline rocks represent the igneous crust of the 

 globe, and iheir frequent stratiform structure is due to agencies 

 in great part anterior to the production of sedimentary rocks. 

 In opposition to this view is that of the neptunist, who, starting 

 from the fact that the elements of an aqueous sediment may, 

 through the action of chemical and crystallogenic forces, pass 

 into new combinations and acquire a new structure, argues not 

 only that all indigenous crystalline rocks have had an aqueous 

 origin, but that the exotic masses themselves represent the last 

 stages of this process of alteration or metamorphosis of sedi- 

 mentary beds. 



Further inquiry into the chemical and lithological composition 

 of the crystailiae rocks, however, brings to light difficulties in 

 the way of both of these hypotheses. To begin with the plu- 

 tonist view, volcanic rocks, both ancient and modern, are more or 

 less nearly related in composition to the gneisses and the stratified 

 greenstones, but we seek in vain among undoubted volcanic or 

 igneous r.jcks for the chemical representatives of the masses of 

 serpentine, olivine, steatite, chlorite, quartzite, magnetite, oligist, 

 and limestone, which appear in the primary formations, and 

 have, all of them, by geologists of the school in question, been 

 regarded as of igneous or plutonic origin. To account for the 

 presence of such rocks among the more or less feldspathic aggre- 

 gates — chiefly gneisses and greenstones — which make up the 

 greater portion of the crystalline formations, three hypotheses 

 have been imagined by plutonists. According to the first of 

 these the earth's interior is a reservoir from which, at times, 

 have been ejected not only basic and ?icidic feldspathic rocks, but 



molten masses of olivine, iron-oxyde, quartz, and limestone. 

 Other geologists of this school have sought to account for the 

 presence of some of these exceptional rocks by a process of so- 

 called segregation, which would assimilate them to endogenous 

 masses. The chemical and geognostical difficulties in the way of 

 both of these hypotheses have, however, led to their general 

 rejection for the third, which supposes these rocks to have been 

 formed by a subsequent local alteration of portions of the ordi- 

 nary plutonic rocks. 



From acknowledged cases of alteration or replacement in 

 mineral species which result in pseudomorphs, and from the 

 more frequent cases of envelopment and of isomorphism, which 

 have been taken for examples of pseudomorphism, it was argued 

 that many species are capable of being changed into others by 

 the loss or addition of certain elements, so that the resulting 

 body often contains no portion of its original constituents. Ex- 

 tending this view from single crystals to rock-masse-, it was 

 maintained that different portions of an igneous or plutonic for- 

 mation, whether basic or acidic, might be transformed into 

 serpentine, chlorite, or limestone. These changes were sup- 

 posed to depend on the action of water, which, aided by 

 hed-t, was regarded as the efficient agent in the local al- 

 terations of plutonic rocks. At the same time the ad- 

 jacent sedimentary strata were supposed to share in these 

 changes, thus giving rise to what have been called contact- 

 formations. In their latest form these doctrines have been well 

 set forth by von Lasaulx and by Knop. This third hypothesis, 

 then, proposes to account for the presence of various exceptional 

 varieties of rock among ordinary plutonic formations by sup- 

 posing that limited portions of these have, at different times, 

 been the subject of very unlike chemical processes, resulting in 

 their complete change into new forms of rock by what has been 

 called pseudomorphic alteration, or metamorphosis. As, how- 

 ever, such a conversion involves a change not only of form but 

 of substance, it has beea more properly designated a meta- 

 somatosis. 



We have next to consider the neptunean view as ordinarily 

 expounded. Ihis, while it accounts by sedimentation for the 

 stratiform arrangement of the crystalline rocks and explains the 

 existence therein of beds of iron ores and limestones, still pre- 

 sents many of the difficulties which are encountered in the plu- 

 tonist view. If, as most neptunists maintain, the great crystalline 

 series have been derived from the alteration of uncrystalline 

 ones, which were not only similar to those of palaeozoic and more 

 recent times, but are, in fact, portions of these which in 

 adjacent regions are still known to us in their original unchanged 

 condition, how are we to explain the genesis of the feldspathic 

 and homblendic rocks which predominate in these crystalline 

 formations? The sandstones and shales from which, in this 

 view, they are supposed to be formed, could never, by them- 

 selves, give rise to the rocks in question, since they are deficient 

 in the ^kalis, and to a greater or less extent in the other ba<;es 

 required for the production of the constituent silicates. To 

 explain their origin, therefore, it becomes necessary to admit 

 the introduction of these various bases from without, and to 

 suppose a series of metasomatic processes more wonderful than 

 those imagined by the plutonist. The latter, by his hypothesis, 

 has already at hand feldspathic and hornblendic rocks which are 

 to be the subjects of metasomatosis, while the neptunist has only 

 the products of their decay. 



In either hypothesis, we have to account for the presence, in 

 the primary formations, of beds and interstratified masses of a 

 great number of exceptional silicated rocks very distinct in com- 

 position from any mechanically-formed sediments, including 

 not only silicates like serpentine, olivine, steatite, chlorite, 

 pinite, garnet, epidote, and hornblende, but of pure orthoclase, 

 as well as of triclinic feldspars. Each of thtse species would 

 require, for its production from any ordinaty igneous or aqueous 

 rock, a separate and independent metasomatic process — involving 

 the addition of certain elements and the abstraction of others — 

 until the whole heterogeneous crystalline series was complete. 

 The author illustrated these views by examples from recent 

 writers, and concluded that the hypothesis of metasomatosis, as 

 maintained both by plutonists and leptunists, supposes the 

 operation in solid rocks of processes of circulation, absorption, 

 elimination, selection, and aggregation scarcely to be equalled 

 in the economy of highly-organised beings, and not easily 

 imagined in the masses of the mineral kingdom. 



Certain geologists suppose the existence of two classes of 

 crystalline stratified rocks;_: the one neptunean, and consisting 



