59 



must first be remarked, that many of the rocks of 

 the transition class are purely chemical in their 

 texture, as the limestones and greenstones, for 

 example, and that many of them contain no 

 organic remains. In the next place, rocks of 

 chemical texture occur among the secondary 

 strata ; and others, bearing marks of mechanical 

 re-composition, are found among the primary. 

 Micaceous schist containing fragments of granite, 

 the primary sandstone, and the conglomerate 

 schists that alternate with quartz rock, all afford 

 examples of this latter occurrence. Innumerable 

 secondary limestones present instances of the 

 former. 



The frequent absence of organic remains from 

 the transition rocks, renders that character of no 

 value in a practical view ; nor can it be con- 

 sidered a natural character, if, in the predomi- 

 nant instances, it is absent. It has moreover been 

 observed, that organic remains were found in 

 limestone alternating with and succeeded by 

 gneiss* ; an event which would render it neces- 



* Description of the Western Islands of Scotland. Article 

 Garvh Island. 



