72 



OUTLINES 0 P* GEOLOGY. 



Molybdena, Wolfram, 

 Titanium, Oxide of tin, 



Magnetic oxide of iron, Plombago. 



163. Granite was at one time considered as the most an* 

 cient, as it was thought the lowest in position of all the rocks. 

 This is however far from being the case, and this group is 

 now known to be intercalated among rocks of sedimentary 

 origin. 



164. Granitoid rocks occur in masses of various dimen- 

 sions, from the size of mountains to that of thin veins. 

 They are found beneath gneiss, and intercalated among the 

 gneisiform rocks of the inferior order. They also traverse 

 the inferior order in veins, and have been found in this form 

 as high as the transition limestone. Thus : near the bank 

 of the Delaware, opposite Easton, is an exposed mass of 

 transition limestone, which is traversed and divided into two 

 parts by a granitoid vein. This group has no limit in the de- 

 scending order, as it often lies beneath gneiss : and we are 

 inclined to consider, as belonging to it, the veins which are 

 found as high as the magnesian limestone. 



ORDER II. PORPHYRITIC. 



165. The distinctive rock of this formation is Porphyry, 

 which is a compact felspar, containing crystals of felspar of 

 a different colour, and crystals of quartz. The colour of 

 the felspar paste is sometimes red, and hence the rock de- 

 rived its name ; sometimes gray, or almost black. 



166. The rocks of this order differ from those of the pre- 

 ceding in being less decidedly crystaline. They have not 

 been found beneath the gneiss, but they traverse, in the^form 

 of dykes, the rocks of the inferior, submedial, and medial 

 orders. 



