Ch. IV.] SCHISTOSE GROUP OF CORNWALL. ^ 



into another, so far from creating confusion, may form the 

 groundwork of a systematic arrangement. We are some- 

 times apt to forget, that the deviations from our most ap- 

 proved systems are not indications of irregularities in the 

 works of nature, but proofs of our imperfect knowledge ; and, 

 instead, therefore, of attempting to reduce all things within 

 the bounds of our limited views, we ought rather to enlarge, 

 or even alter our opinions, so as to adapt them to the phe- 

 nomena of the Creation. Such a line of conduct has been 

 often and very successfully followed ; and, in the case now 

 under consideration, it would probably be found advantageous, 

 though it could not be expected that the application of this 

 principle would, in the first instance, be rewarded with com- 

 plete success. 



In the preceding remarks on the composition of the Cornish 

 rocks, it has been seen that there are, here and there, certain 

 distinct and characteristic rocks : each of these, by gradual 

 transitions through a series of layers or beds (which are 

 sometimes compact, sometimes schistose), passes on either 

 hand into those adjoining. These series may therefore be 

 regarded as constituting genera, of which the central or cha- 

 racteristic varieties, commonly massive, and oftentimes crys- 

 talline rocks, are the types. If this view of the subject be 

 correct, the primary rocks do not occur in a state of con- 

 fusion, but are arranged side by side, according to their 

 affinities, after the manner of a well-regulated museum. 



To return to the description of the rocks of the porphyritic 

 series : the hornblende rocks are, in a great measure, com- 

 posed of compact and schistose greenstones, which repeatedly 

 pass into each other, so that, in traversing them from the 

 granite, they appear to alternate ; this, however, does not 

 take place to any great extent in the length of their beds, for 

 sometimes the massive rocks are insulated in the body of the 

 slate, and vice versa. On approaching towards the granite, the 

 greenstone is generally found to be harder, and becomes more 

 and more siliceous, with quartz veins, which sometimes project 



