OVERLYING ROCKS. 483 



one or other of them is not found. It is next 

 requisite to consider those minerals, which, from 

 their less frequent presence, must be regarded as 

 less essential. 



Of these, hornblende is perhaps the most 

 common. It appears very constant in its colours 

 and general characters ; the only perceptible dif- 

 ference consisting in its proportion and in the 

 magnitude of the crystals. In the syenites it 

 forms but a small proportion of the mass, and is 

 sometimes crystallized in a tolerably distinct 

 manner. In the greenstones, it is commonly 

 more minute and more confused ; and when, 

 in these cases, it becomes excessive and very 

 finely divided, it causes them to pass into ba- 

 salt ; a rock concerning which there are fre- 

 quent differences of opinion, but which is inca- 

 pable of rigid limitation, since, even under this 

 composition, it is but the last of a regularly- 

 graduating series. Where the hornblende is 

 most abundant and most minutely divided, the 

 basalt is most perfectly characterized ; but great 

 differences of aspect follow from variations, not 

 only in this ingredient, but in the colour and 



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