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APPENDIX. No. VI. 



My Dear Sir, British Museum, Nov^StJi, 1817 



In compliance with your desire, I have examined the specimens of 

 rocks that were collected in the late expedition to Congo, and presented bv 

 the Lords Commissioners of the Admiralty to the British Museum. These 

 specimens arc all from the banks of the narrows of the Zaire, and are few in 

 number ; but they suffice to prove that the rock formation which prevails on 

 the banks and islands of the lower parts of the river is primitive, and greatly 

 resembles that beyond the ocean to tlie west : a circumstance which adds to 

 the probability that the mountains of Pernambuco, Rio, and other adjacent 

 parts of South America, were primevally connected with the opposite chains 

 that traverse the plains of Congo and Loango. As vou have already given, in 

 your " General Observations,^' a satisfactory view of the broad geological 

 features of the country along those banks, as far as they could be collected 

 from the narratives of Captain Tuckey, and Dr. Smith, I shall confine myself 

 to a few desultory remarks which offer themselves upon compai-ing the obser- 

 vations of these gentlemen with such of the specimens as have their respective 

 localities affixed to them. 



The specimens from the Fetish rocks exhibit a series of granitic compounds, 

 in which the feldspar predominates : and most of them, especially the fine- 

 grained varieties, contain disseminated a great quantity of minute noble 

 garnets, some of them pellucid, others opaque, and of a reddish brown colour ; 

 and all belonging to the trapezoidal or leucite modification. Similar garnets 

 also abound in the mica slate of Gombae. A few specimens of a siliceous rock, 

 nearly compact, being composed of confluent particles of quai'tz, intermixed 

 with minute scales of mica, are likewise ticketed as obtained from the Fetishes. 

 This insulated group of rocks seems to represent a miniature of the stupendous 

 granitic bulwark, which arises from the plain on the north side of the river 

 Coanzo, near Cabazzo (the capital of Matamba), and of which an account, 

 together with a good representation, has been given by Father Cavazzo. Wo 



