140 



so as to produce a scoriform appearance. Such 

 cells are of various sizes ; but they do not 

 often exceed half an inch in dimensions, 

 and they are also of various forms, spheroidal 

 or elongated. 



AMYGDALOIDAL. 



The amygdaloidal structure may be under- 

 stood from the preceding remarks ; the caverns 

 being filled with a variety of foreign mineral sub- 

 stances, which thus appear like imbedded nodules. 

 These nodules vary in size, from a diameter of 

 some inches down to that of poppy seed. The 

 cavernous and amygdaloidal structures are often 

 united in the same mass ; and both of these are 

 limited to the rocks of the trap, or overlying, 

 family, and to those of volcanic origin. 



AGGREGATE. 



In this structure, fragments of various 

 sizes are united into one mass, or the aggregate 

 is composed of previous rocks broken and 

 reunited. It concludes all the conglomerates 

 and breccias of geologists; and must also be 



