CHAPTER IX. 
Fossils of the Upper Old Red Sandstone much more imperfectly pre- 
served than those of the Lower. —The Causes obvious. — Differ- 
ence between the two Groups, which first strikes the Observer, a 
Difference in Size. — The Holoptychius a characteristic Ichthyolite 
of the Formation. — Description of its huge Scales. — Of its Oc- 
cipital Bones, Fins, Teeth, and General Appearance. — Contempo- 
raries of the Holoptychius. —Sponge-like Bodies. — Plates resem- 
bling those of the Sturgeon.— Teeth of various Forms, but all 
evidently the Teeth of Fishes. — Limestone Band, and its probable 
Origin. — Fossils of the Yellow Sandstone. — The Pterichthys of 
Dura Den. — Member of a Family peculiarly characteristic of the 
System. — No intervening Formation between the Old Red Sand- 
stone and the Coal Measures. — The Holoptychius contemporary for 
a time with the Megalichthys. —The Columns of Tubal Cain. 
Tue different degrees of entireness in which the geologist 
finds his organic remains, depend much less on their age than 
on the nature of the rock in which they occur; and as the 
arenaceous matrices of the Upper and Middle Old Red Sand- 
stones have been less favorable to the preservation of their 
peculiar fossils than the calcareous and aluminous matrices 
of the Lower, we frequently find the older organisms of the 
system fresh and unbroken, and the more modern existing as 
mere fragments. A fish thrown into a heap of salt would be 
found entire after the lapse of many years; a fish thrown 
into a heap of sand would disappear in a mass of putrefac- 
tion in a few weeks; and only the less destructible parts, 
such as the teeth, the harder bones, and perhaps a few of the 
scales, would survive. Now, limestone, if I may so speak, 
is the preserving salt of the geological world; and the con- 
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