OUTLINES OF GEOLOGY. 



47 



superior formations ; and impressions, which are the most 

 usual mode in the lower formations, are rare in these. 



The three first groups have more analogy to each other 

 than either of them has to the fourth ; they are chiefly of 

 marine origin, and the upper rock of one group frequently 

 alternates with the lower rock of that above it. 



106. The cretaceous group is composed of the following 

 rocks : — - 



(1.) White Chalk, in which are imbedded flints often in 

 irregular beds, iron pyrites, and sulphate of strontia. Of the 

 former mineral it is the only geological site. 



(2.) Grey Chalk, or chalk marl, in which are imbedded 

 nodules of horn stone, either dispersed, or in irregular beds, 

 and a rock called fire stone by the English, the Macigno 

 crayeux of Brogniart. The latter is a mixture of sand, chalk* 

 and mica. 



(3.) Green Chalk, which derives its colour from particles 

 of green sand, disseminated through it. This rock contains, 

 imbedded, horn stone, the phosphates of chalk and iron, and 

 pyrites. 



(4.) A Marly Clay. 



107. No remains of terrestrial mammalia have been found 

 in this group. 



Of birds, there are several extinct species. 



Of reptiles, crocodiles, other sauri, and chelonia, all of 

 species and many of genera, no longer living. 



Fish are numerous, and particularly the teeth of sharks. 



Of shells, the families belemnite and ammonite* neither of 

 which is found in modern formations, or those of the superior 

 order, make their first appearance here, with the catillus and 

 inoceramus. On the other hand, trilobites, orthoceree, and 

 producti, do not occur. 



Of radiated animals, echini and asteriae, are numerous. 



Of vegetables, conferva? and algae, are found. 



