4 FOSSIL MAMMALIA. 



of primitive rocks. Be this, however, as it may, we find the 

 sheivy summits of all the grand mountain-chains which inter- 

 sect our continents composed of these primitive strata. The 

 granite almost invariably constitutes the central ridges of these 

 mighty chains, and, singular to relate, it occupies the highes*- 

 and the lowest position in their stratification. That it was 

 forced upwards by some tremendous convulsions of nature, 

 which have shaken this globe to its very centre, is indubitable. 

 The indented ridges, the ragged precipices, the bristhng peaks, 

 by which these primitive chains are always characterized, prove 

 to demonstration the violence which was exerted in their pro- 

 duction. In this respect, they exhibit a decided contrast to 

 those more convex mountains, and undulating ranges of hills, 

 whose mass was quietly deposited by the last retiring sea, and 

 has since remained undisturbed by any violent revolution. 



The lateral ridges of these chains are formed of schistus, 

 porphyry, talc-rocks, &c. which rest on the sides of the granite. 

 Finally, the external ranges are composed of granular marble, 

 and other calcareous strata, but devoid of shells, which rest 

 upon the schistus, and form the last boundary of the empire of 

 mere inanimate matter. We now begin to find, but few in 

 number, and, at intervals, in the transition strata, the earhest 

 animal productions. We find the larger orthocerae, those 

 singular Crustacea, the tribolites, the calymenes, the ogygiae. 

 We find encrinites, numerous species of cornua ammonis, and 

 of terebratulae, belemnites, trigoniae, and other genera, most of 

 which are no longer found in less ancient strata. Terebratulae 

 are found in these ancient strata, in the chalk formations 

 above them, in the shelly hmestone above those, and in the 

 living state ; but the number of species, and even of individuals 

 of this genus, are found to diminish in an inverse ratio to the 

 antiquity of the periods in which they existed. 



We find shelly strata occasionally interposed between beds 

 of granite and other primitive substances, which must have 

 occupied their present situation at a more recent period. That 



