188 ON PRIMITIVE ROCKS. 



imagined* Even sandstone, one of the most abundant 

 of the flcetz-rocks, occasionally occurs in masses, many 

 yards in extent, which individually have a tabular or 

 stratified structure : but when viewed on the great scale, 

 appear to be great massive distinct concretions. These 

 massive concretions, with their subordinate tabular 

 structures, if not carefully investigated, are apt to be- 

 wilder the mineralogist, and to force him to have re- 

 course to a general system of subsidence or elevation of 

 the strata, in order to explain the phenomena they ex- 

 hibit. 



NOTE B. 7. p. 39 & 41. 



On Primitive Rocks. 



As the enumeration of primitive mountain rocks in the 

 text is incomplete, we have judged it useful to give in 

 this note a more full account of them. Primitive moun- 

 tains, in general, form the highest and most rugged por- 

 tions of the earth's surface, and extend in the form of 

 chains of mountain-groups throughout the whole earth. 

 These mountain-groups are generally highest in the 

 middle, and lowest towards the sides and extremities ; 

 and the mountain-rocks of which they are composed, 

 are so arranged, that in general the middle and highest 

 portions of the group are composed of older rocks 

 than the lateral and lower portions. As far as we 

 know at present, granite is the oldest and first formed 

 of all the primitive rocks. This rock is composed 

 of felspar, quartz, and mica, and varies in its struc- 

 ture from coarse to very small granular. It sometimes 

 alternates with beds of quartz and felspar, and is often 

 traversed by cotemporaneous veins of granite, of quartz, 

 and of felspar. The newer or upper portions of the 

 formation contain cotemporaneous masses of porphyry, 

 syenite, hornblende rock, limestone, &c. It frequently 



