GEOGNOSY AND GEOLOGY. — 135 
These gaps, which thus exist in our systems, built up from our knowledge 
of the present living world, are in a great measure filled up by the fresh 
material derived from the study of past races. It is not necessary to the 
character of a fossil remain that actual petrifaction shall have taken place, 
the bones of animals dug up from caves, or found buried in alluvium, being 
truly fossil, and yet possessing much the same composition with recent 
bones. To qualify an object for a place among what are technically called 
organic remains, it is necessary for it to have become extinct at some time 
before the historical age of the world. In some cases, however, as in 
certain species of shells and numerous vertebrata, the same species occurs 
as living, both before and in the present geological era; in this case their 
ancient remains are still true fossils. The science which thus treats of 
long extinct individuals, as well as species, is called Paleontology. 
2. The second point of view from which we look at organic remains is 
the geognostical, inasmuch as they are found in stratified rocks. In these 
they occur in various forms, sometimes as actual remains, and at others 
only as casts or impressions of what once existed. The parts of the 
organism may have vanished, with or without the space vacated having 
been filled up by mineral matter. Again, certain cavities originally 
existing in the object may now be penetrated and occupied by stone. The 
external shell of the animal, when such existed, is sometimes preserved and 
sometimes not. The penetrations of inorganic matter occur very frequently, 
and consist principally of lime, clay, and silex. The penetration of silex is 
exceedingly interesting, inasmuch as it was frequently produced by infusoria. 
These animalcula, with their silicious skeletons, and in infinite numbers, 
probably attacked the soft side of animals, and in dying left their skeletons 
on the spot. This is very conspicuous in some fossil echinide, whose 
calcareous shell incloses a silicious nucleus, which, under the microscope, 
is found to consist of such organisms. Some parts of organic matters have 
also experienced alterations, as well by chemical decomposition as by 
mechanical substitution. An example of such chemical decomposition is 
found in lignite, which originally consisted of wood. A substitution is often 
effected by silex; thus we sometimes see entire trunks of trees of it, and 
not unfrequently one half of a tree or branch replaced by the silex, the 
other half still continuing to be lignite. This silex, probably, in a dissolved 
state, permeated the entire tissue, filling up the spaces left by the removal 
of the organic matter. The pyrites, also, which sometimes lines the 
cavities of fossils, in all probability, infiltrated the mass in the state of 
sulphate of iron, and was subsequently converted into the sulphuret. It is 
well known that decomposing organic matter furnishes powerful means of 
reduction, by abstracting oxygen, to combine with their own decomposing 
particles, which are thus converted into various volatile gases. Another 
mode in which organic bodies occur in the strata of normal rocks, consists 
in the preservation of some portions and not others. Thus the shells of 
mollusca are most frequently found without the animals. The organie 
matter may also have disappeared from these shells, leaving them in a 
calcined state. 
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