28 The Hanging Hills. 



2. Another point worthy of consideration is that if the trap ex- 

 isted as a continuous sheet, it is reasonable to suppose that the rock 

 would be uniform in character, or that the changes in chemical 

 composition, if not in structure, would be gradual rather than ab- 

 rupt. This does not appear to be the case. The rock differs not 

 only in composition but in structure on opposite sides some- 

 times of a narrow ravine, to whatever cause that ravine may be 

 due. That a variety should exist is nothing strange, but some 

 cause for the variety might reasonably be expected. Some- 

 times we find the trap-rock to be homogeneous, and sometimes 

 amygdaloidal ; sometimes very compact, sometimes loosely aggre- 

 gated, and again assuming the porous or vesicular character an- 

 tecedent to amygdaloidal deposits. And if it be true that 

 amygdaloidal deposits usually occur near a cooling surface, this 

 would not account for the presence of amygdaloid in one ridge 

 and its absence in an adjacent ridge, if the existence of the two 

 was due merely to a fracture and fault ; for the cooling must have 

 taken place before the faulting occurred. 



Altogether, the problem is a perplexing one and whatever 

 may come of the discussion, the effect will be to awaken new 

 interest and make us more familiar with these unique formations 

 in our own vicinity. 



