OCR Text |
Show 374 HYPOGENE ROCKS, [Ch.XXVI. district, the lowest of these will be converted into hornblende. schist, while the uppermost may continue in the condition of clay-slate. The te·rm 'Flypogene' proposed fo ·r P1·imary .-If our readers have followed us in the train of reasoning explained in this and the preceding chapter, they must already be convinced that the popular nomenclature of Geology, in reference to the so called 'primary' rocks, is not only imperfect, but in a great degree founded on a false theory ; inasmuch as some gt·anites and granitic schists are of origin posterior to many secondary 1·ocks. In other words, some p1.-imary formations can already be shown to be newer than many secondary groups-a mani· fest contradiction in terms. Yet granite and gneiss, and the families of stratified and unstratified rocks connected with each, belong to one great natural division of mineral masses, having certain characters in common, and it is therefore convenient that the class to which they belong should receive some common name-a name which must not be of chronological import, and must express, on the one hand, some peculiarity equally attributable to granite and gneiss (to the plutonic as well as the alte-red rocks), and which, on the other, must have reference to characters in which those rocks differ both from the volcanic and from the unaltered sedimentary strata. We propose the term' hypogene' for this purpose, derived from {mo, subler, and 'Y'votMx.,, nascor, a word implying the theory that granite and gneiss are bo~h netherformed rocks, or rocks which have not assumed then· ~resent form and structure at the surface. It is true that gneiss and all stratified rocks must have been deposited originally at the surface or on that part of the surface of the globe which is coverc~ by water· but according to the views explained in this ' . d and the foregoing chapter, they could never have acqmre their crystalline texture, unless acted upon by heat under pres· sure in those regions, and under those circumstances where the plutonic rocks arc generated. The term 'MetanWt]Jhic' proposed for sl'l'u.lified prima·ry.- Ch.XXVI.] METAMORPHIC ROCKS. 375 We divide the hypogene rocks, then, into the unstratified or p 1u tom.c , and the altered stratified. For these last the te' rm 'metamorphic' (from !J-Era, trans, and ~J-ogqJ'Yl, form) may be used. The last-mentioned name need not, however, be often resorted to, because we may speak of hypogene strata, hypogene limestone, hypogene schist, and this appellation will suffice to distinguish the formations so designated from the plutonic rocks. By referring to the table (No. I.) at the close of this chapter, the reader will see the chronological relation which we conceive the two classes of hypogene rocks to bear to the strata of different ages. No orde·r of succession in hypogene formations.-When we regard the tertiary and secondary formations simply as mineral masses uncharacterized by organic 1·emains, we perceive an indefinite series of beds of limestone, clay, marl, siliceous sand, sandstone, coal, and other materials, alternating again and again without any :fixed or determinate order of position. The same may be said of the hypogene formations, for in these a similar want of arrangement is manifest, if we compare those occurring in different countries. Gneiss, mica-schist, hornblende- schist, quartz rock, hypogene limestone, and the rest, have no invariable order of superposition, although, for reasons above explained, clay-slate must usually hold a superior position relatively to hornblende schist. We do not deny, that in a particular mountain·chain~ a chronological succession of hypogene formations may be recognized, for the same reason that in a country of limited extent there is an order of position in the secondary and tertiary rocks, limestone predominating in one part of the series, clay in another, siliceous sand in a third, and so of other compounds. It is probable that a similar prevalence of a regular order of arrangement in the hypogene series throughout certain districts, led the earlier geologists into a belief, that they should be able to fix a definite order of succession for the various members of this great class throughout the world. |