136 



LYELL'S ELEMENTS OF GEOLOGY. 



Origin of the Metamorphic Rocks. 



since the first introduction of organic beings into this planet, but 

 even long after many distinct races of plants and animals had 

 passed away in succession. 



The doctrine respecting the crystalline strata, implied in the 

 name metamorphic, fnay properly be treated of in this place ; 

 and we must first inquire whether these rocks are really entitled 

 to be called stratified in the strict sense of having been origin- 

 ally deposited as sediment from water. The general adoption 

 by geologists of the term stratified, as applied to these rocks, 

 sufficiently attests their division into beds very analogous, at 

 least in form, to ordinary fossiliferous strata. This resemblance 

 is by no means confined to the existence in both of an occa- 

 sional slaty structure, but extends to every kind of arrangement 

 which is compatible with the absence of fossils, and of sand, 

 pebbles, ripple mark, and other characters which the meta- 

 morphic theory supposes to have been obliterated by plutonic 

 action. Thus, for example, we behold ahke in the crystalline 

 and fossiliferous formations an alternation of beds varying 

 greatly in composition, colour, and thickness. We observe, for 

 instance, gneiss alternating with layers of black hornblende- 

 schist, or with granular quartz, or limestone; and the inter- 

 change of these different strata may be repeated for an indefi- 

 nite number of times. In the like manner, mica-schist alter- 

 nates with chlorite-schist, and with granular limestone in thin 

 layers. 



As in fossiliferous formations strata of pure siliceous sand 

 alternate with micaceous sand and with layers of clay, so in 

 the crystalline or metamorphic rocks we have beds of pure 

 quartzite alternating with mica-schist and clay-slate. As in the 

 secondary and tertiary series we meet with limestone alternating 

 again and again with micaceous or argillaceous sand, so we 

 find in the hypogene, gneiss and mica-schist alternating with 

 pure and impure granular limestones. 



Tt has also been shown that the ripple mark is very com- 

 monly repeated throughout a considerable thickness of fossil- 

 iferous strata, so in mica-schist and gneiss, there is sometimes 

 an undulation of the laminse on a minute scale, which may, 

 perhaps, be a modification of similar inequalities in the original 

 deposit. 



In the crystalline formations also, as in many of the sedi- 

 mentary before described, single strata are sometimes made up 

 of laminse placed diagonally, such laminse not being regularly 

 parallel to the planes of cleavage. 



