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Show CHAPTER XXV. On the rocks usua1 1 Y te r mecl' Primary '-Their relation to v. olcanic and sedim. en. tary forma t1• ons-The ' n1 · rimary' class divisible in• to strahfi•e tl a•n d unstrahfied -Unstratl'f ie d roc k s called Plutonic-Granite vem. s. -Theu v.a nous forms and mm· e1·al compos1' t1' 0n-Proofs of their igneous ongm-Gmmtes of the . same character pro d uce d a t suC cessive eras-Some of these newer . than certum. fos· s1· h· ferous s t rn. t a-D'I ffic u lty of determining the . age of parhcular gramtes- Distinction between the volcanic and the plutome rocks-Trappean rocks not separab I e f 1·om the vO lcanic-Passa,.o, e from t.r ap into granite-Theory of the Ol·'J g·m or gram't e at every n1· eriod from the earliest to the most recent. ON THE ROCKS COMMONLY CALLED PRIMARY. WE shall now treat of the class of rocks usually termed' pri. mary,' a name which, as we shall aftet:wards show, i~ notal ways app 1.t ea bl e, since the formations so d. esignated sometimes belo. ng to different epochs, and are not, m every case, mo:c ~n~1~nt than the secondary strata. In general, however, th1s dms10n of rocks may justly be regarded as of higher antiquity than the oldest secondary groups before described, and they may, therefore, with propriety be spoken of ~n these conclud~ng chapters, for we have hitherto proceeded m our retrospective survey of geological monuments from the newer to those of more ancient date. In order to explain to the reader the relation which. we con· ceive the rocks termed 'primary' to bear to the te:t1ary and secondary formations, we shall resume that general view of the Ponent parts of the earth's crust of which we gave a slight com b' · 1 sketch in the preliminary division of our su ~ect m tle 2nd chapter *. . . VVe there stated that sedimentary formations, contammg oro-anic remains, occupy a large part of the surface of our c~n-t . 0 men ts , bttt that here and there volcanic rock. s occur, bdr eakm· g through, alternating with, or covering the sedimentary eposits, "' See above, p. 8. Ch, XXV.] PLUTONlC ROCKS. 353 so that there are obviously two orders of mineral masses formed at the surface which have a distinct origin, the aqueous and the volcanic. No. 84. I a, Formations called primary (stratified aud unstratified). b, Aqueous formations. c, V olcauic rocks. Besides these, however, there is another class, which cannot be assimilated preci~ely to either of the preceding, and which is often seen underlying the sedimentary, or breaking up to the surface in the central parts of mountain-chains, constituting some of the highest lands, and, at the same time, passing down and forming the inferior parts of the crust of the earth. This class, usually termed' primary,' is divisible into two groups~ the stratified and the unstratified. The stratified consists of the rocks called gneiss, mica-schist, argillaceous-schist (or clayslate), hornblende-schist, primary limestone, and some others. The unstratified, or Plutonic, is composed in great measure of granite, and rocks closely allied to granite. Both these groups agree in having, for the most part, a highly crystalline texture~ and in not containing organic remains. Plutonic rocks.-The unstratified crystalline rocks have been very commonly called Plutonic, from the opinion that they were formed by igneous action at great depths, whereas the volcanic, although they also have risen up from below, l1ave cooled from a melted state upon or near to the surface. The theory conveyed by the name Plutonic is, we believe, correct. Granite, porphyry, and other rocks of the same family, often occur in large amorphous masses, from which small veins and dikes are sent off, · which traverse the stratified rocks called 'primary,' precisely in the manner in which lava is seen in Some places to penetrate the secondary strata. Granite Veins.-We find also one set of granite veins intersecting another, and granitiform porphyries intruding VoL, III, 2 A |