480 OVERLYING HOCKS. 



attention to differences of a more constant and 

 essential nature. 



Another source of confusion has arisen from 

 dwelling too strongly on peculiarities of struc- 

 ture as essential distinctions ; it being a common 

 practice with collectors of specimens, to reject 

 those of which the characters are not very de- 

 cided ; although these are often the most im- 

 portant, as marking the transitions which occur 

 in nature* Thus the porphyries have been im- 

 properly separated from the simple rocks which 

 form their bases, although the transition to the 

 porphyritic structure is imperceptible. An at- 

 tempt has been made to remedy these defects by 

 adopting an arrangement more consonant to 

 nature ; but it is unnecessary to explain it in 

 these preliminary remarks, as the reasons may be 

 deduced from an examination of the Synopsis. 



The predominant substance in the members of 

 this family is a simple rock ; of which, indurated 

 clay or wacke, may be placed at one extreme, 

 and compact felspar at the other ; the interme- 



