OUTLINES OF GEOLOGY. 



53 



122. True coal makes its first appearance in the joints of 

 this group ; but the layers are so thin as not to be worth 

 working. 



HI. MEDIAL ORDER. 



Carboniferous group, Deiabeche ; Terrains Abyssiques Rudiment air es, Houiller^ 

 Carboniferes, Brogniart; Lower Secondary Formations. 



123. The medial order is made up of four series : 



1. Millstone grit and shale ; 



2. The coal measures ; 



3. Carboniferous limestone ; 



4. Carboniferous sand-stone. 



124. The rock called millstone grit is composed of angu- 

 lar fragments of quartz and felspar, held together by a hard 

 argillaceous cement. The shale is distinguished from clay- 

 slate by being an aggregate of minute particles, instead of 

 being wholly made up of a single mineral species. These 

 two rocks alternate with each other without any determinate 

 order. The position of this group may be either above or 

 below the coal measures. Although no doubt can exist of 

 its stratification, this mode of formation is sometimes difficult 

 to detect, in consequence of the thickness of the beds of grit, 

 and its resemblance to some of the rocks not stratified. 



125. This group rarely contains organic remains. The 

 few which have been found in it are vegetable, similar to 

 those of the coal measures. 



126. It is sometimes very rich in metallic ores, which oc- 

 cur in detached spots, in nodules, in druses, and in interca- 

 lated beds, but never in veins. These ores are galena, blende, 

 pyrites, carbonate of copper, sulphuret of mercury, specula? 

 iron, calamine, manganese,, and chrome. With them are 



5* 



