MacCulloclfs System of Geology. 365 



of the ablest and most Ions-sighted of those now enframed in 

 this gigantic labour. 



If we have succeeded in explaining ourselves, it will be very 

 clear that we would wish to consider geology as having two 

 essentially distinct parts, — Descriptive Geology and the Philoso- 

 phy of Geology. In fact, the mere description of the earth's 

 crust is essentially a branch of natural history, whilst the spe- 

 culation becomes one of physics the moment that the element 

 of time is introduced, and we begin to investigate the history 

 and causes of any species of change to which it may be sub- 

 ject, whether past or future. This, then, is a clear line of de- 

 marcation ; and the two branches of the science are so neces- 

 sarily distinct, and require such a very different class of acquire- 

 ments and habits of thought for their respective prosecution, 

 that we wonder they should so generally be considered as one 

 and the same, or as distinguished by a line so purely metaphy- 

 sical that the one cannot be practically cultivated without the 

 other. For ourselves, we have no hesitation in considering the 

 physics of geology as the most interesting and important de- 

 partment of the science, without any wish to undervalue the 

 practical labours of those who furnish data to found and to 

 test speculations. But arc we to infer that men wanting the 

 technical knowledge requisite to the complete natural histori- 

 cal description of a phenomenon, should be the less able on 

 that account to deduce correct inferences respecting its causes 

 and the legitimate conclusions to which it leads ? Oris the re- 

 lation of the mere observer the less to be relied on because he 

 may be incapable of drawing from it all the inferences it may 

 warrant? We would say, much the reverse. The theory of 

 the one and the facts of the other are only the more worthy 

 of confidence ; the one is unable to bend his fuels to accommo- 

 date his theory ; the other has no theory to support. To tike 

 an example : A physical geologist wishes to know the epoch of 

 elevation of a mountain range ; and suppose tiiat from the di- 

 rection of the chain he suspects from hypothesis that it was 

 elevated at one period rather than another; he finds one rock 

 formation lying upon the flanks of the range in a way indica- 

 tive of having been raised along with it, whilst another is su- 

 perimposed unconformably in horizontal strata. He is en- 

 titled to infer from dynamical principles that the epoch he re- 



