OVERLYING ROCKS. 477 



depth as to emulate the soft claystones, and thus 

 to mislead an observer respecting their true cha- 

 racters. 



The minuter structures are, the small cavern- 

 ous, the amygdaloidal, and the porphyritic ; and, 

 on these two latter, no remarks are necessary, 

 except to observe that they may be combined in 

 the same rock. But the former is worthy of par- 

 ticular notice, on account of the exact resem- 

 blance which it bears to that which occurs in the 

 scoriform lavas ; the cells being often elongated, 

 or contorted, and glazed by a vitreous varnish, 

 while they are also sometimes partially filled 

 with the substances that enter into the amygda- 

 loids. Thus the cavernous passes into the 

 amygdaloidal structure, and it is also sometimes 

 found united to the porphyritic. , 



So many rocks, of different character and 

 composition, are contained in this family, that 

 no general remarks on their texture and compo- 

 sition can be offered ; while the confusion which 

 has hitherto reigned among them, renders it 

 necessary to enter into details much more minute 



