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Show 364 AGE OF PLUTONIC ROCKS. [Ch.XXV. contrary, that it has been heated, and still continues f~r centuries, or thousands of years, at a red heat, the vem may acquire a highly crystalline texture. 'l'he great pressure of a superincumbent mass, and exclusion from contact with air or water, are probably the usual conditions necessary to produce the granitic texture; but the same may sometimes be superinduced at a slighter distance from the surface by slow refrigeration, when additional sup• plies of heat check, from time to ti.me, the cooling process and cause it to be indefinitely protracted. If, for the reasons above alluded to, we conceive it probable that plutonic rocks have originated in the nether parts of the earth's crust, as often as the volcanic have been generated at the surface, we may imagine that no small quantity of the former class has been forming in the recent epoch, since we suppose that about 2000 volcanic eruptions may occur in the course of every century, either above the waters of the sea or beneath them *. We may also infer, that during each preceding period, whether tertiary or secondary, there have been granites and granitiform rocks generated, because we have already discovered the monuments of ancient volcanic eruptions at almost every period. In the next chapter we shall endeavour to show, that in consequence of the great depths at which the plutonic rocks usually originate, and the manner in which they are associated with the older sedimentary strata of each district, it is rarely possible to determine with exactness their relative age. Yet there is reason to believe that the greater portion of the plutonic formations now visible are of higher antiquity than the oldest secondary strata. We shall also endeavour to point out, that this opinion is by no means inconsistent with the theory that equal quantities of granite may have been produced in succession, during equal periods of time, from the earliest to the most modern epochs. • See vol. i. c~1a1>· xxii. CHAPTER XXVI. On the stratified rocks usually called t primary '-P ~ f: • •• • COOI9 rom the d1sposd10n or their. stra. ta that th.e y. were originally deposited f:r 0 m wa t er-Alt ernab·o n of betls varym~ 1D composition and colour-Passage of gneiss into granite-Alteration of sedimentary strata by trappean and granitic ·''k 1 " • • ul es- Illerence as to the ongm of the strata called ' primary ,-Conversion of argi' lla ceous m· to horn. bhmde schist-The term 'Hy1Jogene' J>roposed as a su 1)8 t1't u t e 1"0 r pr·i mary- ' Metamorphic' for.' stratified primary' rocks-No regular order of succession of hypogene formation. s-Passa.g e from the metamo rph 'lC t o the se dJ' mt.>ntary strata-Cause of the h1gh relahve antiquity of the v· 'bl h ti · • • • lSI e ypogene ormabons -That anhqmty con. s1sten.t w.i th the hypothesis tha t th ey h ave been pro-duced at each success1ve penod m equal <1uantities - G r· e a t vo 1u me of l typogene rocks ksupposed to have been formed since the Eocene period-Concluding remar s. ON THE STRATIFIED ROCKS CALLED ' PRIMARY.' WE stated in the last chapter, that the rocks usually termed 'primary' are divisible into two natural classes, the stratified and ~he unstratified. 'l'he propriety of the term stratified, as applied to the first-mentioned class, will not be questioned when the rocks so designated are carefully compared with strata known to result from aqueous deposition. Mode of sfrati.ficalion.-If we examine gneiss, which consists o: the same materials as granite, or mica-schist which is a bmary compound of quartz and mica, or clay-slate, or any ~th.er member of the so-called primary division, we find that 1t IS made up of a succession of beds, the planes of which are, to a certain extent, parallel to each other, but which freq: ently deviate f~o~ parallelism in a manner precisely analooous to that exlnbited by sedimentary formations of all ages. Tl~e resemblance is often carried farther, for in the crystalline series we. find beds composed of a great number of Jaye1·s placed dwgonall y, as we have shown to be the case in the |