520 



NA TURE 



{Sept. 27, 1888 



as of parts of the crenitic layers themselves, in the form of erup- 

 tive rocks, forming not only plutonic masses, but those which we 

 have designated as pseiuioplutonic— that is to say, masses of 

 crenitic origin which present the geognostic characters of plutonic 

 rocks. Such are apparently the trachytes and the truly eruptive 

 granites. Eruptions of these two classes of rocks seem to have 

 been rare in the more ancient periods, but in later times they 

 have played an important part in the transfer of mineral matters 

 from the depths to the surface of the globe, while at the same 

 time the crenitic activity has progressively decreased. Without 

 questioning the effect of the slow contraction through the 

 secular cooling of the heated anhydrous and solid nucleus of the 

 globe, we believe that the diminution of volume of its more 

 superficial and hydrated portions by the crenitic process, a* well 

 as by plutonic eruptions, has played a very important part in 

 geological dynamics. 1 



§ 6. According to the hypothesis just set forth, it follows that 

 the production alike of the crenitic and the plutonic rocks, as the 

 result of the transformations of a primitive material presumed to 

 be of igneous origin, has been subjected to constant, regular, 

 and definite laws. It shows, in fact, a mineralogical evolution 

 which has determined the order, the composition, and the suc- 

 cession of the crenitic masses of the terrestrial crust, as well as 

 the composition of the plutonic masses of the various geological 

 periods. In the study of the successive groups of crenitic rocks 

 we must take into account the intervention in the crenitic pro- 

 cess alike of the soluble and the insoluble products of the aerial 

 decomposition both of more ancient crenitic rocks and of plutonic 

 masses, as well as the effects, both direct and indirect, of the 

 products of organized beings. It results from the influence of 

 all these secondary agencies which have intervened in the course 

 of the crenitic process, that the fundamental granite, as the most 

 ancient crenitic r^ck, presents chaiacters of uniformity and of 

 universality which do not reappear in the less ancient crenitic 

 terranes. These, in fact, already begin to show indications of 

 a passage to the new order of things, and were thus, in the 

 language of the Wernerian school, called Transition rocks. 



As a farther result of this mineralogical evolution in the history 

 of the crenitic rocks, we find that certain aluminiferous silicates 

 rarely met with at a given period, at length become more 

 abundant and finally predominate. For this reason it follows 

 that in the mineral kingdom, as in the organic kingdoms, 

 generalizations which have for their object chronological classi- 

 fications, should be founded upon the character of a group taken 

 in its integrity, and not upon the characters of exceptional 

 species. For the rest, ii is to be remarked that non-aluminiferous 

 species, such as the pr >toxyd silicates, quartz, carbonate of lime, 

 and oxides of iron are found, with small variations, in the crenitic 

 masses, whether indigenous or endogenous, alike of earlier and 

 of later periods. 



It is evident that the operations of solution and of aqueous 

 deposition, as well as those of decomposition and sub-aerial 

 decay, went on in the Primary and Transition periods under 

 geographical conditions which did not differ greatly from those 

 of the Secondary and Tertiary periods. The marks of erosion, 

 of contemporaneous movements, and of deposition in discordant 

 stratification are met with at different horizons in the indigenous 

 terranes of the Primary as well as in those of the Secondary ages ; 

 offering iu both cases local and accidental interruptions of the 

 normal order of mineralogical development. 



§ 7. The various granitic, quartzose, and calcareous vein- 

 stones, including metalliferous lodes, not less than the veins and 

 geodes of zeolitic minerals, are examples of endogenous masses 

 formed by the crenitic process. The production of zeolites and 

 of other silicates by the action of thermal waters, and the 

 formatiun of zeolitic species in the deep-sea ooze, are examples 

 of the same crenitic action continued to our own time. As is 

 shown by the studies of the action of our modern thermal 

 springs, the surrounding solid matters co-operate with those in 

 solution in the production of new mineral species. We must 

 not overlook the part which is often played by infiltrating waters 

 in producing local transformations in sediments, thereby giving 

 rise to the production of crystalline species in the midst of 

 detrital rocks. Pressure alone appears in certain cases to pro- 

 duce similar results, all of which cases are often insisted upon 

 in support of the application of the metamorphicand metasomatic 

 hypotheses to the origin of the primary rocks. 



1 Besides the removal < f all the silica and alumina found in the crenitic 

 rocks must be added the diminution of porosity in the protoplas nic mass and 

 the probable formation of more condensed species than those or.ginally 

 contained therein. > 



The granitic veins, composed essentially of orthoclase and 

 quartz, which are found not only among gneisses and mica- 

 schists, but among basic plutonic rocks alike of Palasozoic and 

 of Mesozoic age, 1 help us to understand the conditions which 

 in times of greater crenitic activity gave rise to the production 

 of the gneisses and the fundamental granite, both of which, 

 according to our hypothesis, are essentially neptunian and 

 crenitic in their origin. These same indigenous and endo- 

 genous crenitic rocks have furnished the greater part of the 

 materials for the Secondary rocks. We have already indicated 

 concisely, in § 4, our explanation of the origin of the true 

 plutonic rocks, as the result of modifications which have taken 

 place in the midst of the protoplasmic mass. 



§ 8. We mu-t not lose sight of the important part played by 

 water in plutonic and volcanic phenomena, nor the fact that it can 

 exist under strong pressure, at high temperatures, in combination 

 with silicated rocks. From this union there result hydrated 

 compounds, which are mope fusible than the anhydrous rocks, 

 and which are decomposed in the transformations that take 

 place during the cooling, with diminution of pressure, which 

 accompanies the eruption of these materials. The water thus 

 set at liberty may be disengaged in the form of vapour, and with 

 it certain other volatile matters which are met with in volcanic 

 emanations. In other cases, however, m der a high pressure 

 still maintained, and at a temperature above the critical point of 

 vaporization, the water may be liberated in the state of a dense 

 polymeric vapour, holding in solution, in accordance with late 

 observations, mineral matters, which, through cooling, are at 

 length deposited either from the vapour itself or from the liquid 

 resulting from its condensation, in the form of crystalline species. 

 Superheated aqueous vapours may thus play a part closely akin 

 to that of thermal waters, and one which must be regarded as 

 itself belonging to the crenitic process. 



The greater part of the questions here noticed have already been 

 discussed in detail by the author in his volume entitled "Mineral 

 Physiology and Physiography " (Boston, 1886). especially in the 

 three chapters on the Origin, the Genetic History, and the 

 Decay of Crystalline Rocks (pp. 68-277.) 



II. 



§ 9. In another chapter of the volume just mentioned the 

 author treats of the History of l're-Cambrian Rocks (pp. 

 402-25), and endeavours to resume in a few pages the results 

 of his attempts through a period of forty years to arrive at a 

 subdivision and a nomenclature of these terranes, which com- 

 prise both the Primary and the Transition systems of Werner. 

 It must suffice for the present to indicate in a succinct manner 

 the conclusions already reached. 



I. Laurentian. — Under this name, proposed and adopted 

 by the author in 1854, is included the ancient gneissic terrane 

 met with in the Laurentide and the Adirondack Mountains, as 

 well as in parts of the great Atlantic belt, and in the Rocky 

 Mountains in central North America. To this same series the 

 author has also referred the similar gneisses of Great Britain 

 and of Scandinavia, as well as the ancient or central gneiss of 

 the Alps. Beginning with our first studies in Canada in 1847, 

 we indicated the existence in this ancient gneissic system of two 

 subdivisions, the lower being described as consisting of granitoid 

 gneiss (to be confounded with the fundamental granite), to 

 which succeeds (in discordant stratification) another gneissic 

 series, also granitoid, and frequently hornblendic, with which 

 are intercalated quartzites and crystalline limestones, often with 

 serpentine. These two subdivisions, which we may provisionally 

 call Lower Laurentian and Upper Laurentian, have been de- 

 scribed respectively as the Ottawa gneiss and the Grenville 

 series. To prevent any misconception, it should be noted that 

 the name of Upper Laurentian was for a time given by Logan 

 to the terrane subsequently designated Labradorian, and after- 

 wards dorian. It is therefore by a mistake that some have 

 wished to retain as the designation of the upper division of the 

 Laurentian terrane, the name of Middle Laurentian. 



1 We have elsewhere described the granitic veins inclosed in the diabases 

 which themselves traverse the Ordovician limestones of Montreal in 

 Canada. These veins, having sometimes a thickness of three decimetres, are 

 coaisely crystalline and drusy, and. besides quanz and orth.clase, contain, as 

 accidental minerals, sodaltte, nephelite, cancrinite, amphibole, acmite. 

 biotite, and magnetite. Veins composed essentially of pink orthoclase and 

 quartz, often ace mpanied by zeolitic minerals, are found in similar con- 

 ditions inclosed in the diabases which are contemporaneous with the 

 Mesozoic sandstonesof Hoboken, near New York In both cases the endo- 

 genous and crenitic origin of the granitic veins does not admit of any doubt. 

 See for details the author's ''Mineral Physio'ogy and Physiography" 

 (Boston, 1886), pp. 121-37. 



