1821.] Neighbourhood of Okehampton, Devon . 1 65 



the rocks of Sourton, Sticklepath, and other portions, appa- 

 rently of the same formation, surrounding this extremity of 

 Dartmoor, most of which abound in hornblende, it may fairly, 

 under the present state of our knowledge and nomenclature, be 

 termed greenstone. Whether it may not become expedient 

 hereafter to assign a separate class to those rocks of which com- 

 pact felspar forms the principal ingredient, or whether a careful 

 pursuit of the experiments of Cordier and others may not pave 

 the way to a more accurate subdivision of the obscurer members 

 of the greenstone and trap families, are questions which I wil- 

 lingly leave to your more acute and better informed readers. 

 To such as have the opportunity, I would further recommend 

 the careful examination of the rocks immediately incumbent on 

 the granite of Dartmoor, especially those which occur between 

 that rock and the limestone so frequently occurring in its neigh- 

 bourhood. 



The following should have been added to the memoranda 

 on the Red Marie of Devonshire : 



The only rock hitherto ascertained to be subordinate to the 

 red marie of Devonshire is the amygdaloid, which has been 

 observed in various parts of its extent, but most conspicu- 

 ously in the neighbourhood of Thorverton and Silverton. This 

 rock was first noticed more than 20 years back, and not inaccu- 

 rately described (if my memory serve me aright) by Dr. Maton, 

 in his tour through the western countries. 



Its general aspect is that of a granular mass, somewhat 

 loosely compacted, of a purplish-brown colour, more or less 

 intense (given most probably by the oxide of manganese in 

 which it abounds). In this paste are imbedded, or rather inter- 

 mixed, in such quantities as to form a very considerable part of 

 the whole mass, minute portions of calcareous spar, mica, or 

 chlorite, in a state of semi-disintegration, and indurated clay 

 (lithomarge ?), sometimes ringed by copper, and sometimes by 

 manganese. This latter substance, as well as the calc spar, 

 frequently traverses the rock in small veins. The cells of the 

 amygdaloidal portions are filled or lined with brown oxide of 

 manganese, with calc spar and a coarse jasper. The nodules of 

 the latter are not remarkable either for their size or beauty. The 

 character of the rock is so obscured by this abundant admixture 

 of substances apparently adventitious as to render it very 

 difficult to pronounce with any certainty as to its essential con- 

 stituents. These we should, I apprehend, in the present state 

 of our knowledge, assume to be granular or earthy felspar, and 

 one or more of the following : hornblende, augite, bronzite, or 

 hyperstene, probably the second of these. My specimens do 

 not afford distinct indications of any of them. The more com- 

 pact portions fuse before the blowpipe, sometimes into black 



