B 



SECONDARY ROCKS. 



mary and traiisilion class are the principal repositories of metallic 

 ores, but in Europe they contain lew saline or inflammable miner- 

 als.^" In South America, according to Humboldt, sulphur and bi- 

 tumen exist in considerable quantities in rocks denominated pri- 

 mary. 



Rocks of the transition class are not universally interposed be- 

 tween the primary and secondary rocks, for in some situations the 

 transition series are entirely wanting. Thus in passing from Lyons 

 to Clermont, in the centre of France, I observed the regular coal 

 strata resting upon beds of sand, clay, and rounded stones which im- 

 mediately cover granite. 



Secondary Rocks. — The lower series are almost all distinctly stra- 

 tified ; they consist chiefly of sand-stone, soft argillaceous slate call- 

 ed shale, and beds of coal and iron-stone. Many of the secondary 

 strata of this class abound exclusively in the fossil remains of veget- 

 ables, analogous to ferns, palms, and reeds ; while the rocks in the 

 former or transition class, contain almost exclusively the remains of 

 marine animals. This change in the nature of the fossil remains in 

 the two classes of rocks, indicates an important change in the condi- 

 tion of the globe, prior to the deposition of the lower series of sec- 

 ondary strata. The transition rocks were evidently formed under 

 the &ea, some of the beds being composed almost entirely of the ex- 

 uviae of madrepores and encrini, but the terrestrial plants whose re- 

 mains abound in the lower secondary strata, must have grown on 

 land, from which the ancient ocean had retired, and the strata which 

 contain them were probably deposited at the bottom of rivers or fresh 

 water lakes, as marine organic remains seldom, if ever, occur in 

 them. The upper series of secondary strata again indicate another 

 important change of the surface of the globe. The prevailing beds 

 in this series are stratified limestone with beds of clay shale and sand- 

 stone interposed. The limestone has generally an earthy texture, 

 and very rarely partakes of the hard and crystalline character of the 

 lower limestones. The fossil remains in the upper secondary strata 

 are, with some exceptions, those of marine animals, but of different 

 genera or species from those in the strata below them. It is in the 

 upper secondary strata that we first meet with remains of saurian or 

 lizard-shaped animals, some of which were of immense size. The 

 co-existence of dry land, at the period when most of the upper sec- 

 ondary strata were deposited, is, however, proved, by the occasion- 

 al occurrence of terrestrial fossil plants, and the bones of fresh-water 

 and amphibious reptiles, such as the crocodile and tortoise. 



Another important fact respecting the upper series of secondary 

 strata is, that they appear to have been formed, not only under dif- 

 ferent circumstances from the lower, but after a long interval, during 



* Except we comprise the regular coal formation in the transition series 



