526 



NATURE 



{Sept. 27, 1888 



associated amphibolites and cipolines. Above this first division 

 chloride and sericitic mica schists are developed, alternating 

 occasionally with amphibolitic layers. This second stage is 

 succeeded by a series which also comprises hornblendic and 

 augitic {comes vertes) schists, but includes, further, the first 

 detrital deposits. At every horizon there is a gradual passage 

 from the one stage to the other. The first detrital deposits 

 alternate with sericitic and chloritic schists ; and even as far up 

 as in the Cambrian, large bands of felspathic schists, which can 

 scarcely be distinguished from the more ancient gneisses, are 

 developed in connection with the intrusion of granite. 



The primitive rocks are, as first pointed out by the author, 

 injected and penetrated by ancient eruptive rocks. This 

 phenomenon is al o to be observed in the earlier detrital schists. 



Rolled pebbles and fragments of gneiss, mica-schist, &c, 

 have been repeatedly found in the granitic and granulitic gneisses 

 of various localities. The author's own observations lead him 

 to compare these phenomena with those in which rounded balls 

 have been inclosed in a truly eruptive granite. In numerous 

 cases, in which fragments of gneisses have been enclosed in other 

 gneisses, he has always been able to prove that the enclosing 

 rock is much more felspathic than the inclosed fragments. 



These facts cannot, therefore, be advanced in support of the 

 detrital origin of true gneisses. 



(2) Mode of Association of the Component Minerals. — The 

 mineralogical composition of the gneisses and of the schistose 

 basic rocks associated with them, is nearly identical with that 

 of the granular eruptive rocks ; and all the types of the older 

 eruptive rocks have their representatives in the schistose series. 



A great analogy therefore exists between the natural forces 

 instrumental in the production of the two series. 



Speaking generally, the older eruptive rocks are rigorously 

 homogeneous over vast areas : fragments of these rocks are 

 everywhere comparable to one another. This homogeneity is 

 reproduced in the schistose series ; but it is, so to speak, periodic, 

 and one must first know the orientation before comparing 

 fragments taken from a distance. 



The structure of the gneisses presents a series of successive 

 crystallizations, accompanied by mechanical phenomena and a 

 cementing of the dislo&ated components. The author, while 

 seeing in these phenomena the traces of a series of metamor- 

 phic actions, followed by the injection of foreign material, does 

 not wish to deny the additional intervention of secondary 

 mechanical actions. But, whatever theoretic explanation be 

 adopted, the facts are well established, and irreconcilable with 

 the assumption of a preliminary mixing of the magma of the 

 schistose rocks, and therefore with the hypothesis of a primordial 

 origin. 



The author then proceeds to demonstrate at some length that 

 the intimate structure of the gneisses is identical with that of 

 sedimentary schists modified by contact metamorphism, and 

 finally injected by eruptive rocks. 



Microscopic studies have disclosed the minute liquid inclusions 

 contained by the quartz of the gneisses. Zirkel and Kalkowsky 

 have made the interesting observation that the streams of in- 

 clusions are restricted to the central portions of the quartz-grains 

 and are not prolonged to the periphery ; and De Lapparent 

 adduces this fact as a proof that the grains have not been derived 

 from a pre-existing rock. But this argument is overthrown 

 by the fact that the quartz-grains in the Cambrian micaceous 

 schists, which are of indisputably detrital origin, present exactly 

 the same phenomenon. It admits, moreover, of a very simple ex- 

 planation. These quartz-grains, of clastic origin, have undergone 

 subsequent enlargement by the assimilation of secondary quartz, 

 which tends also to give them an exterior crystalline form. This 

 secondary quartz is poor in liquid inclusions, and encloses scales 

 of black mica and other minerals. 



General Considerations and Hypotheses on the Origin of the 

 Primitive Rocks. — Among the hypotheses advanced to explain the 

 origin of gneiss, the author discusses the two that have found 

 the most general acceptance. The first, which is now somewhat 

 abandoned but has the merit of perfect clearness, makes the 

 gneisses the result of a kind of conflict between water and the 

 primary molten magma of the earth. The other explanation, 

 which is more vague, accords to the gneisses a sedimentary 

 origin. They are the deposits of a kind of supersaturated sea, 

 which precipitated on to its floor the successive crystalline bands 

 which characterize the gneisses. Note that this hypothesis 

 presupposes a floor — an unknown substratum. 



(1) Geologists originally supposed that the first substratum was 



formed by the granites which are found cropping out over such 

 vast areas. Detailed studies have shown, however, that the 

 granites are younger than the gneisses which they traverse, 

 inject, and displace. Even the most ancient among them are at 

 least younger than the first detrital schists. 



It is therefore to the gneisses, distinctly banded and alternating 

 in their lower beds with mica-schists, that this mixed origin — 

 this rdle d'ecumes pnmordiales— must be attributed. 



Has this substratum of the terrestrial crust ever been seen in 

 the most disturbed regions? 



Cordier supposed that terrestrial refrigeration was constantly 

 increasing, in the downward direction, the thickness of the first 

 solid crust. If we could descend through the earth's crust, we 

 should pass successively through rocks of increasing basicity until 

 we should find, enveloping the still incandescent nucleus of 

 impure iron, a rock analogous to lherzolite. 



A serious objection to this is the fact that a descending order 

 of basicity is not borne out by the stratigraphical relations of the 

 gneisses. Lherzolite is found erupted through the primitive 

 rocks ; and the basic peridotites are intercalated moderately high 

 up in the gneissic series. 



From the purely speculative point of view it is improbable 

 that the first products of consolidation did not receive a thorough 

 mixing, rendering the rock homogeneous, and preventing the 

 formation of those numerous micaceous membranes so charac- 

 teristic of the primitive rocks. If these first products were acid, 

 as there is reason to suppose, the first substratum must have 

 constituted a massive and homogeneous granite. It is on a floor 

 of this kind that the precipitation of the atmospheric waters 

 must have prepared the elements of the first detrital rocks — the 

 first arkoses. 



(2) The second explanation — the successive crystallization of 

 bands of gneiss from the waters of a universal sea— encounters 

 similar difficulties. It appears to the author irreconcilable 

 with the structure of the gneissic rocks. The continuous mem- 

 branes of mica, and the almost vein-like appearance of the 

 quartz and felspar, do not accord with the notion of con- 

 cretionary deposits that this hypothesis requires, supposing the 

 supersaturated liquid to have been in a state of perfect tran- 

 quillity. If, on the other hand, we suppose that there existed 

 local agitations due to the unequal distribution of high temper- 

 atures, the remarkable periodic homogeneity of the gneisses 

 becomes inexplicable. 



From a consideration of these facts and hypotheses the author 

 arrives at the conclusion that the veritable and primary sub- 

 stratum of the terrestrial crust is not visible ; that this substratum 

 has undergone much alteration ; finally, that the so-called primi- 

 tive rocks are a complex of eruptive rocks, later than the gneisses, 

 and of rocks which are really detrital, but which have undergone 

 excessive metamorphism. 



The eruptive rocks, by which the primitive rocks have been 

 injec.ed, are later than the beginning of the Cambrian. They 

 were produced in extraordinary abundance in the later por- 

 tion of" this period : granites, diabases, diorites, norites, and 

 lherzolites. 



In discussing the primary causes of the eruption of these 

 rocks, the author mentions that Lehmann and others of the 

 German school, are inclined to seek them in the partial trans- 

 mutation into heat of the mechanical work performed during 

 the intense periods of contortion undergone by the earth's crust. 

 The author himself refers them to manifestations of the internal 

 heat of the globe, the great earth-movements having simply 

 effected the ascension and injection of the eruptive magmas. 



NOTES. 

 By the death of Mr. Jameson on the Upper Congo, science 

 has lost a most promising young naturalist. The collections 

 made by him some years ago in Borneo were never described, 

 but we believe that in that island Mr. Jameson met with many 

 species of birds since obtained by other travellers. His expedi- 

 tion to Mashoona Land resulted in the discovery of some 

 interesting new species of birds, and an elaborate paper was 

 written on his collection by Captain Shelley in the Ibis for 

 1882. A small number of birds has also been sent by him from 

 the Aruwimi River to his friend Mr. Bowdler Sharpe, who has 

 been waiting for further collections before writing an account of 



