164 Proceedings of Philosophical Societies. [Aug. 



■everal. Dr. Hamilton's Commentary on the first part of 

 Rheede's great work, has already appeared in the Transactions 

 of the Linnean Society, vol. xiii. part. ii. p. 474. 

 The Society then adjourned to the 4th of November next. 



GEOLOGICAL SOCIETY. 



May 16. — A letter was read, from Henry Heuland, Esq. For. 

 Sec. Geol. Soc. addressed to the President, " On the Matrix of 

 the Diamond." 



In this letter Mr. Heuland describes two specimens which he 

 laid upon the table of the Society. The first of these, from 

 Abbaete in Brazil, was a conglomerate of oxide of iron, with 

 small waterwom quartz pebbles, containing a diamond. This, 

 which is called Cascalhao, Mr. Heuland believes to be of allu- 

 vial origin. The other specimen from Pereira, in Brazil, which 

 Mr. Heuland received from Baron d'Eschwege, was a very small 

 brilliant dodecahedral diamond, surrounded by skorodite or 

 cupreous arseniate of iron in a gaugue or matrix of massive oxide 

 of iron (Werner's brown ironstone.) This oxide of iron, accord- 

 ing to Baron d'Eschwege and Alexander Caldcleu^h, Esq. 

 forms veins or beds 25 feet deep resting on chlorite schist in the 

 mountains near Pereira. That it is the true matrix, of at least 

 the Brazilian diamond, appears confirmed by the locality where 

 diamonds have not before been discovered, by its being accom- 

 panied by the arseniate of copper, and by the difference of this 

 oxide of iron from that in the Cascalhao, which is either earthy, 

 granular, or in water-worn particles. 



June 6. — A paper was read containing remarks on Sections 

 presented by tne Rivers Isla, Melgum, Proson, and S. Esk, in 

 the County of Forfar, with some general Observations on the 

 Geology of that County, accompanied with specimens. By 

 Charles Lyell, Esq. Sec. GS. 



The country which formed the principal subject of this com- 

 munication is situated on the southern flank of the Grampians ; 

 it is occupied by old red sandstone, greywacke, and argillaceous 

 schist, with their associated porphyries. The strata are clearly 

 exposed by the rivers that cut through them. They are very 

 highly inclined, and dip for the most part towards the south. 



The old red sandstone may be described as consisting of two 

 formations of sandstone, with a formation of conglomerate of 

 great thickness interposed between them. An extensive forma- 

 tion of felspar porphyry occurs in the lower part of the conglo- 

 merate, and it is from the broken and rolled fragments of this 

 porphyry that the conglomerate is for the most part composed. 

 Between the porphyry and the conglomerate, a rock prevails of 

 a mixed character, which seems intermediate between the two, 

 and which it is difficult to describe or account for. The lower 

 red sandstone, which is beneath the conglomerate, is in many 

 parts seen to be traversed by a mass or dyke of greenstone, 



