OVEKLYING ROCKS. 519 



term appears to have been sometimes used in a 

 different sense by foreign writers. 



These comprise many of the rocks, both simple 

 and compound, of the preceding catalogue, as 

 bases, in which are imbedded nodules of various 

 adventitious minerals. The prevailing amygda- 

 loids, however, have bases consisting of the harder 

 varieties of indurated clay B, Division First. 



Occasionally the porphyritic and amygdaloi- 

 dal structures are combined, as already remarked, 

 producing a set of incidental varieties which 

 it lias not been thought necessary to describe. 

 The amygdaloids may contain one, or more mine- 

 rals, and hence also arise numerous varieties. 



Their aspects further vary according to the 

 magnitude of the nodules, and the proportion 

 which these bear to the base. These nodules are 

 sometimes solid ; at others they do not fill the 

 cavity which contains them. 



In some cases, two, or more, minerals are con*- 

 tained in the same cavity, as agate and calcareous 

 spar. At times, considerable portions of an amyg- 

 daloid contain cavities, without minerals, as well 

 as others partly filled, together with solid nodules. 



