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PART II. 



CHAP. XXIII. 



Classification of Rocks. 



THE more general facts in Geology having thus been 

 examined, under a separation, which, if not perfect, 

 seems all that a System at present admits, I must 

 proceed to the Second Part of this work, or to de- 

 scriptions of the materials which constitute the acces- 

 sible earth; though these often entangle enquiries of 

 a general nature. Unfortunately, also, they must often 

 repeat things urged before ; because they comprise 

 the Evidences of what has preceded. To produce a 

 System from established materials, is a very simple 

 task : to endeavour to build one out of materials un- 

 acknowledged, unknown to others, or controverted, 

 under a perpetual toil through evidences and arguments, 

 is a very different labour: as it is the fate of the writer 

 to pioneer that way which others soon follow; without 

 acknowledgement or eifort, yet not without claiming 

 and receiving the praise. But I must premise some 

 remarks on the Classification of these materials ; since, 

 besides its practical utility, this constitutes an im- 

 portant branch of Geological Philosophy. 



VOL. II. F 



