1821.] Neighbourhood of Okehampton, Devon. 163 



line, and exhibiting frequently even in hand specimens very 

 singularly tortuous forms.* in one spot this rock has been 

 excavated under the hope of finding tin. The trial appears to 

 have been unsuccessful. The Deads afforded specimens of coarse 

 dark-brown garnet, associated with calcareous spar and axinite, 

 the latter much disguised by its incorporation with the rock 

 which serves as its matrix, and which, as well as its imbedded 

 minerals, is much charged with oxide of iron, and considerably 

 increased in weight. It is also rendered, perhaps by these 

 extraneous admixtures, much more fusible.t This rock is suc- 

 ceeded by thick and nearly vertical beds of a veiy compact 

 greenstone (B), exhibiting for the most part rather the character 

 of granular felspar tinged by the intimate admixture of horn- 

 blende. The face of Meldon Hill formed by this rock (on the 

 left bank of the stream) is precipitous and highly picturesque. 

 Here the felspathic veins appeared to cease, but the garnet and 

 axinite still occurred in small contemporaneous veins, and were 

 somewhat more distinctly characterized than in the slate A. We 

 found traces also of epidote. Upon this greenstone rests, at a 

 very high angle, the limestone (C) of Meldon Quarry interstratified 

 with beds of (hornblende ?) slate more or less compact, and occa- 

 sionally of granular felspar nearly free (as far as the eye can 

 judge) from any foreign admixture. When the alternation of 

 this with the dark hornblende slate has taken place on a small 

 scale, it affords very handsome specimens for the cabinet. £ The 

 limestone itself is black, of an earthy texture, good quality, and 

 extensively worked. It contains, as far as our observation went, 

 no traces of organic remains.^ In the beds of granular felspar, 

 we observed minute rifts coated with a mixture of calcareous 

 spar and a mineral of a light-brown colour, and considerable 

 lustre, beautifully arranged in stelliform groupes composed of 

 numerous minute prisms radiating from a common centre. If it 

 be not a variety of epidote (to which species, from its compara- 



» I subjoin a list of some among the more obvious : their general resemblance to the 

 Elvans of the Cornish Killas will be immediately seen. 



1 . Granular felspar and quartz, with some traces of silvery mica. 



2. Same aggregate, more crystalline, and with a greater portion of mica. 



3. Same, with numerous plates of silvery mica (mica talcite ?) and specks of dark 

 violet coloured fluor spar. 



4. Quartz and tourmalin. 



■f It has long ago been advised that those specimens of simple minerals only should 

 be selected for chemical examination, or analysis, which are free from all admixture 

 either of the rock which forms their matrix, or of any other foreign ingredient. The 

 name caution should be carefully extended to rock specimens which have their charac- 

 ters in many instances as much or more disguised and altered by the intimate admix- 

 ture of imbedded minerals. 



t In some places, the hornblende is distributed in patches, giving the rock a cloudy 

 appearance ; its particles being still too minute to be identified as hornblende even with 

 IUM, 



§ This limestone is rapidly acted upon by dilute muriatic acid. Thus treated, it 

 yields a considerable residuum, consisting of carbon and an earthy matter, readily fusi- 

 ble by the blowpipe into a semi-transparent globule, of a white colour, which may be 

 considered, perhaps, as felspar in a state of minute division. 



M2 



