130 Notes, principally Geohgwal, on Southern India* [Jan. 



observe dykes of a silicious rock*, penetrating the gneiss, and in some 

 places overlapping it for a short distance on either side the dyke. This 

 rock, like the trap rocks of Bijapore, has a tendency to the schistous 

 structure, and splits like them, when struck by the hammer,! nto angular 

 frfigments, the superficies of which, exhibit similar dendritic appear. 

 anoe-<, but more perfect. The fissures by which the rock splits (the 

 dry cracks of European miners), as may be discovered by a micros- 

 cope, and often even by the naked eye, are pre-existent in its structure, 

 and resemble those in basalt, and other substances, which have cooled, 

 after being greatly heated. The dendritic appearances in igneous 

 rocks have been ascribed to the infiltration of metallic oxides, and 

 with more show of reason to electric currents, circulating through the 

 pores of the rock; but in some instances, particularly in rocks of ac- 

 knowledged aqueous origin, they assume so much the character of 

 mosses and ferns, that it would be matter of some interest, if botanists 

 would institute a minute enquiry into the forms usually seen in each 

 rock, with a view of ascertaining the question of their organic origin. 

 In rocks of a decidedly igneous character, the arborescent appearances 

 are generally far less perfect than those in limestones, and other rocks 

 of deposition ; and will be probably found, as just remarked, to be 

 occasioned by the metamorphic powers of electric currents, acting upon 

 metallic particles disseminated in the substance of the rock. The 

 beautiful arborescent outlines in mocha stones are now supposed 

 to arise from mineralized plants of the cryptogamous class, and Dr. 

 MaccuUoch has detected both in them, and the moss agates, aquatic con- 

 fervas, unaltered both in colour and form, and coated with iron oxide. 

 An onyx agate belonging to the Earl of Powis, contains the chrysalis of 

 a moth ; and the red colour, observable in this class of minerals, has 

 been discovered by M. Turpin to be owing to an insect, Protococ- 

 ens Kermesinus, in form of scminules, or red globules. M. Gaulty de 

 Chaubry found, that when powdered cornelians were mixed with black 

 oxide of copper in a porcelain retort, and heated to redness, carbonic 

 oxide and carbonic acid were disengaged; and some pyro-acetic acid, 

 and pvrogenic oil formed : proving that the colouring matter of the 

 mineral is of an organic nature. Apologizing for this digression, I shall 

 rest content for the present, by calling attention to this curious 

 subject, expressing at the same time, my strong belief, that many 

 appearances in transition and secondary rocks, hitherto considered 



• This rock is probably identical with that described as a hornstone by Buchanan, and 

 a siticious slate by Dr. Benza; the fissures imparting a schistous character to the rock, 

 do not resemble the laminae ,of deposition. 



