94 STRUCTURAL AND FIELD GEOLOGY 
calcareous sandstones, however, are not infrequently highly 
fossiliferous—and this is especially the case when the 
sandstones occur in beds alternating and _ interosculating 
with dark carbonaceous or lighter coloured calcareous shales. 
Conglomerates are generally unfossiliferous, or, if fossils 
are present, these are usually more or less rolled and water- 
worn. For example, we may obtain, in some Carboniferous 
and Jurassic conglomerates, worn fragments of the trunks 
and branches of trees—but the more delicate twigs and 
leaves are absent. So, again, in gravels and conglomerates 
of Pleistocene and Recent age, only the more resistant large 
bones and teeth of mammals are ever met with, and they are 
often rolled and broken. There are exceptions to every rule, 
however, for, now and again, tolerably well-preserved shells do 
occur in conglomerates. 
Volcanic Tuffs—I\n certain bedded volcanic tuffs fossils 
occur, but this is not common. Plant-remains have even been 
encountered in the coarse tuffs and agglomerates that occupy 
thejthroats or necks of certain ancient Carboniferous volcanoes 
in Scotland. Probably these represent trees, etc., which grew 
upon the slopes of the old cones after the volcanoes had 
become extinct. More rarely still, charred fragments of trees 
have been met with enclosed in the lower portion of an 
ancient lava. 
Schistose Rocks.— It need hardly be said that these rocks 
are usually destitute of organic remains. Nevertheless, fossils 
are occasionally present in schists, as in certain metamorphic 
Silurian rocks in the neighbourhood of Christiania, and in 
the highly crystalline schists of Liassic age which enter into 
the structure of the Central Alps. 
Fossils differ much not only in regard to the state of 
preservation of their internal structure, but also of their 
external form. In many cases, they have been much com- 
pressed—what were formerly cylindrical branches, for example, 
have often been flattened, so as to give lenticular sections 
when cut across. In limestones, marly shales, and calcareous 
sandstones, shells, corals, etc., usually retain their original 
shapes ; while in argillaceous shales, fossils of all kinds are 
apt to be more or less flattened—a rule, however, which has 

many exceptions. In clay-slates and rocks which have 

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