Ch. IX.] VEINS IN PRIMARY ROCKS. 167 



minute and faithful delineation of a vein, it cannot be very 

 surprising that the task of definition has been so imperfectly 

 performed. This may appear to be a startling assertion, but 

 it is made advisedly; and, indeed, the reason of this de- 

 ficiency is obvious. 



Hitherto we have been indebted almost entirely to practical 

 men for information on this subject ; and it cannot be ex- 

 pected, from this source, to obtain such minute and circum- 

 stantial details, such nice and delicate discriminations, as are 

 requisite for scientific purposes : indeed, the nature of their 

 pursuit is, in some measure, incompatible therewith, their 

 object being only to obtain the ore, and that as expeditiously 

 as possible. It may be hoped that this deficiency, as far at 

 least as Cornwall is concerned, will be soon supplied, by 

 the labours of Mr. Kenwood, who has nearly completed his 

 survey of the mines. 



In the meantime, however, it is necessary to give a general 

 idea of the Cornish veins, because they afford some important 

 evidence, which will be required in the following discussion. 

 The descriptions about to be offered have been drawn from 

 veins which occur on the sea-shore, in quarries, and in similar 

 natural and artificial sections; from these sources much 

 valuable information may be gained concerning the position 

 of veins, the nature of their connection with the containing 

 rocks, and many of the appearances produced by the inter- 

 ference of one set of veins with another. These exhibitions, 

 however, are most satisfactory, when the view is confined to a 

 horizontal section or ground plan ; whilst the phenomena 

 displayed by veins, during their descent, are best investigated 

 in the mines. 



When a section of the primary rocks is closely inspected, 

 it is found that no individual rock continues pure and uni- 

 form in its composition for any considerable extent ; and far 

 more commonly each of its constituent blocks or concretions 

 exhibits a striped or variegated appearance, on account of 

 numerous irregular veins which intersect its mass. These 



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