FOSSILS 93 
which are termed fossils, but any recognisable trace of their 
former existence—any impressions or tokens left behind 
them—whether it be footprints, tracks, or trails, burrowings, 
castings, or coprolites, or even the markings traced on sedi- 
ment by the waving to-and-fro of seaweeds, etc.—are all 
equally fossils. 
Kinds of Rock in which Fossils occur.—As a rule, 
the best preserved fossils are met with in the finer grained 
sedimentary rocks, as in marls, limestones, clay and shale, 
and fine argillaceous or calcareous sandstones. 
Calcareous Rocks.—Argillaceous limestones and marly 
shales are often highly fossiliferous, and the fossils are 
usually well preserved. But pure limestones, which have 
become more or less crystalline, frequently appear to be 
poor in organic remains, so that when a fresh fracture of 
the rock is obtained, few or no traces of any structure may 
be visible. On surfaces which have been for some time 
exposed to the weather, however, fossils not infrequently 
project in bold relief—the limey matrix in which they are 
embedded offering less resistance to atmospheric action. 
The same phenomena characterise many dolomitic lime- 
stones. 
Argillaceous Shales.—Not infrequently these are rich in 
fossils—their impervious character having doubtless tended 
to the preservation of the remains. Some shales, however, 
are very barren, or the few fossils present may be included in 
nodular concretions of calcium-carbonate, siderite, or other 
substance. 
Sandstones are not so frequently fossiliferous as shales, for 
which there are at least two reasons. First, a sandy sea-floor, 
owing to frequent or constant movement of the sediment, is 
not favourable to sedentary forms of life, and is therefore 
avoided by organisms which cannot shift for themselves. An 
ordinary siliceous sandstone might therefore be expected to 
be somewhat barren. Second, the permeable character of 
sandstones must favour the subsequent passage of percolating 
water which so frequently dissolves and removes organic 
bodies. Massive thick-bedded quartzose sandstones and 
red sandstones are, as a rule, singularly poor in organic 
remains. Certain thin-bedded argillaceous and thick-bedded 
