OUTLINES OF GEOLOGY. 



51 



116. The oolitic group occupies a considerable portion of 

 Europe. It has been best studied in England, and in the 

 Jura Mountains, from the last of which the French geolo- 

 gists have named it. It is wholly wanting in the United 

 States, or at least has not yet been discovered ; the rocks of 

 oolitic structure found in this country belonging to a more 

 ancient formation. 



1 17. The succeeding group, which with Omalius we shall 

 call Triasic, is named by the English, and some of the Ger- 

 man mineralogists new red sand-stone. It is generally made 

 up of the following rocks : 



Psammite, a rock composed of quartz, held by an argilla- 

 ceous cement. The cement is in this group charged with 

 oxide of iron, which gives it a red colour. The quartz may 

 be of any colour, but is often ferruginous. 



Red marl, or as it is called in Jersey, red shale ; 



Bituminous, and carboniferous shales; 



Conglomerate, composed of pebbles of quartz, felspar, and 

 rounded fragments of older rocks, cemented by a red clay ; 



Red clay ; 



A sand-stone composed of aggregated fragments of quartz, 

 probably occurs in the group, but is rare ; 

 Gypsum ; 

 Rock salt; 



Manganesian lime-stone. 



118. The upper strata of marl and sand-stone often alter- 

 nate with the lower strata of lias, and thus there is no deci- 

 ded line of division between the two groups. The gypsum, 

 and petiole ularly the rock salt, are of value ; and, in addi- 

 tion, there are subordinate beds of lignite, hydrated oxide 

 and carbonate of iron, the sulphates of strontia and baryta, 

 and manganese. The lignite is accompanied with fine im- 

 pressions of fossil vegetables. In the magnesian limestone are 

 found metallic veins, whose gangue is usually the sulphate of 



