Ch. XXYIL] supposed period OF INVERTEBRATE ANIMALS. 585 



naturally expect that other proofs of imconformability will hereafter he 

 detected at more than one point in so vast a succession of strata. 



The mineral character of the Upper Laurentian differs, as we have 

 seen, from that of the Lower, and the pebbles of gneiss in the Huronian 

 conglomerates are thought to prove that the Laurentian strata were 

 already in a metamorphic state before they were broken up to supply 

 materials for the Huronian. Even if we had not discovered the 

 Eozoon, we might fairly have inferred from analogy that as the quartz- 

 ites were once beds of sand, and the gneiss and mica-schist derived 

 from shales and argillaceous sandstones, so the calcareous masses, from 

 400 to 1000 feet and more in thickness, were originally of organic 

 origin. This is now generally believed to have been the case with the 

 Silurian, Devonian, Carboniferous, Oolitic, and Cretaceous limestones 

 and those nummuhtic rocks of tertiary date which bear the closest 

 affinity to the Eozoon reefs of the Lower Laurentian. The oldest 

 stratified rock in Scotland is that called by Sir E. Murchison " the 

 fundamental gneiss," which forms the whole of the Island of Lewis in 

 the Hebrides. On this gneiss, in parts of the Western Highlands, the 

 Lower Cambrian and various metamorphie rocks rest unconformably. 

 It is conjectured that this ancient gneiss of Scotland may correspond 

 in date with part of the great Laurentian group of North America. 



ON THE ABSENCE OF VERTEBRATA IN ROCKS BELOW THE UPPER 

 SILURIAN. 



Supposed Period of Invertebrate Animals. — We have seen that in 

 the upper part of the Silurian system a bone-bed occurs near Ludlow, 

 in which the remains of fish are abundant, and amongst them some 

 of highly organized structure, referred to the genera Onchus and 

 Pteraspis. We are indebted to Sir R. Murchison for having first 

 announced, in 1840, the discovery of these ichthyolites, and he then 

 spoke of them as " the most ancient beings of their class." In the 

 third edition of his classical work,* he has reverted to the opinion 

 formerly expressed by him, observing that the active researches of the 

 last twenty years in Europe and America " had failed to modify that 

 generalization." He also adds : " The Silurian system therefore may 

 be regarded as representing a long period in which no vertebrated 

 animals had been called into existence." 



In the same year (1859) in which this remark was hazarded, the 

 discovery of the Pteraspis, mentioned by us at p. 555, in the Lower 

 Ludlow rocks, carried back our knowledge of the existence of fish one 

 step farther into the history of the past. But it is still a fact well 

 worthy of notice, that no remains of vertebrata have yet been met with 

 in any strata older than the Lower Ludlow. 



Yvlien we reflect on the hundreds of Mollusks, Echinoderms, Trilo- 



• Siluria, p. 268, 1859. 



