496 GEOLOGY. 



and therefore lias less lustre and lower translucency than the primi- 

 tive limestone. It is frequently traversed by veins of calcareous 

 spar, and often exhibits in the same bed various tints and shades of 

 beautiful colours. Some varieties are conglomerated, forming the 

 brecciated marble of artists, and others contain fossil shells and co- 

 rals. 



" 4. Trap. This rock, like that of the primitive class, is prin- 

 cipally composed of hornblende, and is sometimes associated with 

 felspar, forming transition greenstone. 



" 5. Granite, Syenite, and Porphyry These have the same com- 

 position as in the primitive class ; and, independent of the charac- 

 ters derived from their mass, and their particular imbedded mine- 

 rals and veins, are distinguished by the greywacke, with which they 

 are associated. 



" 6. Gneiss and Mica-Slate These rocks occasionally occur as- 

 sociated with the greywacke and other members of this class. 



" 7- Serpentine and Quartz Rock. These very nearly resemble 

 those of the primitive class, but are distinguished from them by their 

 connection with greywacke," &c. 



III. SECONDARY ROCKS. This extensive class of rocks 

 in their geological position rest immediately on those of the 

 transition class ; but when these are wanting succeed the 

 primitive series. The hills of secondary districts are lower, 

 rounder, with acclivities more gentle, and fewer abrupt cliffs, 

 than in the preceding series. The valleys which occur are also 

 less deep. Nearly all the secondary formations are more or less 

 distinctly stratified, and the strata are more frequently horizontal 

 than in the older rocks. The regularity of direction of the stra- 

 ta, so remarkable in the two preceding classes, has not been ob- 

 served in the present. Secondary rocks are particularly dis- 

 tinguished by the variety and abundance of fossil organic re- 

 mains contained in them. In the older formations of the 

 series, remains of oviparous quadrupeds or lizards are met 

 with, while in the newer members a gradual approximation is 

 found in tbe animal remains to the more perfect classes. Coal, 

 of which one species occurs in tbe primitive and secondary rocks, 

 is found in the secondary class in great abundance. The most 

 abundant metals in this series are iron, lead, and copper ; zinc 

 in the form of calamine ; mercury in the form of cinnabar, and 

 cobalt. Rock salt first makes its appearance in this series of 



