9^ THE GEOLOGIST. 



more like the reality if the books were supposed to be placed slantingly 

 on a table, each more or less overlapping another. Still this would 

 not be quite the truth; and, if we regarded the books as con- 

 BCC'itive volumes of one work, some piled variously over a bottom 

 row of slanting volumes, and others differently disposed and 

 heaped about the table, we should make the example approach 

 still more nearly to the actual manner in which the various strata have 

 been deposited slantingly on the shores of the ancient continents, in 

 various large but limited districts, and during certain successive 

 periods only. In this super-stacking of the books we shall have some- 

 thing like the unconformable superposition which has taken place where, 

 after the denudation or slicing-off of the prominences of the ancient 

 surface, portions of land have gone down and the waters of the 

 ancient seas have deposited fresh strata of mineral matter on 

 the older beds at different angles of inclination. Between the 

 elevation of the first beds, during the time they were dry land and 

 the time of their submergence, and that of the deposition of the newer 

 beds, it is evident that other deposits might be going on in other parts 

 of the globe ; and that, thus, though the ages of the beds, like the num- 

 bering of the volumes, might be continuous, yet, as on the whole table the 

 consecutive volumes might be far apart from each other, so the rocks of 

 consecutive ages might be deposited within far distant areas of our planet. 



The sedimentary rocks were, of course, derived from pre-existing 

 materials. They are formed, in almost all cases, of the aggre- 

 gated particles of sand, mud, and clay originally derived from the 

 disintegration and wearing-down — by the action of the waves, the 

 weather, atmospheric influences, and other causes — of the primordial 

 granites. In some, as in mica-schist,"^ we may still without difficulty 

 recognize the unchanged scaly plates of glittering mica, just as they 

 sparkled in the derivative mass ; in others, the origin of the clay of 

 the decomposed felspar and of the sands of the pulverised quartz 

 is not so easily determined without the skill of the microscopist 

 or the chemist. Lime alone %eemB added to the principal primitive 

 ingredients, flint, iron, and clay. But lime, soda, and potash are known 

 to exist largely in the felspars and micas of the granites ; and we are 

 not yet sufficiently aware of the existing relative proportion of lime to 



* Some mica-schists have undoubtedly been developed by metamorphic 

 agencies. — S. J. M. 



