94 
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- 
secutive 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 uncoyiformable 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 seems 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. 
