OVERLYING ROCKS. 499 



The analysis of basalt however, yields soda, 

 even according to those by whom the term is 

 limited to this variety. But hornblende does not 

 contain soda, and the specimens thus analysed 

 must therefore be of a different nature ; probably 

 containing compact felspar. If the term were 

 actually to be thus limited, such specimens do 

 not rank under it. 



a. Massive, irregular. 



b. Laminar. 



c. Columnar, or prismatic. 



This basalt occurs both in veins and in masses. 

 It appears to be sometimes laminar in the former, 

 independently of the action of the weather. The 

 columnar and laminar structures are sometimes 

 combined, as they are in the claystones ; and the 

 latter is either parallel to the axis of the prisms, 

 or at right angles to it. 



The columns of basalt are sometimes jointed, 

 in the various ways already described ; but that 

 structure is not limited to this rock, as already 

 shown. Mineralogists have long since rejected 

 the antient distinction of the columnar structure 

 as characterizing basalt. 



K K 2 



