520 OVERLYING ROCKS. 



When such cavities are numerous, these rocks 

 bear a considerable resemblance to some of the 

 scoriform or cellular lavas. 



The transition from the simple rocks to the 

 amygdaloids, is sometimes imperceptible ; as a 

 rock, of which the prevailing character is simple, 

 may contain occasional nodules of the different 

 minerals. 



It may easily be conceived, that, under all 

 these circumstances, the variety of amygdaloids 

 is very great ; but those which contain the seve- 

 ral zeolites and calcareous spar, are apparently 

 the most abundant. 



It would produce a long and little useful cata- 

 logue were all the varieties of amygdaloid to be 

 enumerated in detail, according to the relative 

 characters of the bases, the nature of the minerals 

 imbedded, and the different combinations in 

 which they exist. It nds therefore been held suf- 

 ficient to enumerate the minerals in a catalogue ; 

 and they are placed, as far as possible, with some 

 regard to the relative frequency of their occur- 

 rence. The collector of rocks may form his own 

 arrangement on that basis. 



