252 



THE YOUNG NATURALIST. 



have been formed from other and still more | 

 antique rocks, is evident from their stratifica- 

 tion. 



They have been so altered that fossils are 

 very rare and imperfect. 



A species of Foraminifera, called Eozoon j 

 Canadense has been found in Canada and j 

 Europe, traces of plant life are to be met 

 with. 



During the Laurentian epoch, the tempera- 

 ture of the air must have been enormously 

 high, because of the thinness of the crust 

 surrounding the fluid centre of the earth. 

 This heat would induce constant evaporation 

 of the water, causing the air to be surcharged 

 with aqueous vapour, and so having a foggy 

 appearance. The Laurentian rocks are over- 

 layed by the Cambrian. From the presence 

 of ripple-marks in this latter formation, we 

 are led to suppose that it was deposited on 

 the shores of lakes and rivers. Fossils of the 

 class Polyzoa are found, viz. : Oldhamia radiata 

 and 0. antiqua, and also several species of a 

 crustacean animal, called the Trilobite. As its 

 name implies, it was divided into three lobes 

 or segments. The only parts of the animal 

 that have been preserved in a fossil state, are 

 the hard shelly coatings, which were com- 

 posed of two layers ; the outer and thinnest 

 coat having sometimes a number of small 

 excresences distributed over its surface. 

 Some species were e} r eless, but the greater 

 number possessed a facetted cornea. The 

 presence of these compound eyes is sufficient 

 to prove that the animals did not live in deep 

 water, where they would not have required such 

 a complicated structure. These Trilpbites are 

 very variable in size, some species being about 

 half an inch in length, while others again 

 reach the extraordinary dimensions of two 

 feet. 



The next formation is termed the Silurian, 

 Trilobites are much more abundant in Silurian 

 rocks than in the previous formations, some 

 species (Calymene) being often rolled up like a 

 woodlouse. Fish remains also occur. 



After the deposition of the Silurian strata, 



the bed of the ocean where the British Isles 

 now are, seems to have slowly risen, until it 

 appeared above the surface, large lakes being 

 enclosed therein. These lakes would be at 

 first salt, though they afterwards contained 

 fresh water. The surrounding banks of 

 Silurian age becoming worn away by the 

 method before explained, the waste material, 

 falling to the bottom, formed the strata of 

 Old Red Sandstone. 



Vegetable remains consisting of tree-ferns, 

 club mosses, and others, are found in these 

 rocks ; shells are rare, but a crustacean 

 occurs, somewhat resembling the lobster, 

 called the Pterygotus. Fishes, in great beauty 

 and perfection, are found in abundance ; one 

 species named Asterolepis sometimes measur- 

 ing thirty feet in length. 



The Devonian rocks, which in some parts 

 of England overlie the Silurian, abound with 

 beautiful corals, In order to exhibit these 

 forms to their best advantage, the stone must 

 be cut and highly polished. All rocks, as a 

 rule, when freshly broken, have a whitish 

 appearance, which effectually prevents their 

 contents from being clearly distinguished. 

 Therefore, when such a stone is under 

 examination, it is usual to damp the surface 

 with the saliva or a moist sponge, when the 

 fossils, if any, become immediately plainly 

 visible. 



Insects make their first appearance in these 

 Devonian rocks, those of Canada having 

 yielded a species of Libellula. 



The Devonian formation is succeeded by 

 the Carboniferous series, comprising the Car- 

 boniferous Limestone and the Coal Measures. 



The numerous sea-lilies and corals which 

 abound in the former of these two formations, 

 denote that it was deposited in deep sea ; 

 hence after the old Red Sandstone and 

 Devonian were deposited, the land sank, and 

 then rose in the form of shallow flats, on 

 which sub-tropical plants flourished in luxu- 

 riance. The fossils of the Coal Measures are 

 of terrestial origin, comprising cryptogamic 

 plants, fresh-water shells, insects, and am- 



