CHAPTER XII. 



DESCRIPTIVE GEOLOGY. 

 THE NORTHWESTERN DISTRICT. 



The Amacura River. — The Amacura River forms the western 

 boundary of the colony, and the structure of the district it traverses 

 has been described by Mr. H. I. Perkins, F.G.S., who travelled on it 

 during the delimitation of the Venezuelan Boundary in the year 

 1900-1901. The following is the description given in his report 

 published in 1901 :- — 



" No rocks are met with in the lower portions of the Barima and 

 Amacura Rivers. In the Amacura the first rocks are found a few 

 miles below the Falls of San Victor, which, like those of the falls, are 

 composed of a coarse granite, with small veins and stringers of quartz 

 running through them. Above the falls, again, for many miles the rocks 

 are granite, and in places large boulders occur in the river bed. The 

 banks are composed chiefly of the decomposition-products of granite 

 and gneiss, and gneiss is found at Juanita and Middle Camp. Between 

 these two places extensive dykes of hornblende-rocks occur, giving 

 where they have weathered the characteristic red and ochreous clays 

 stained by the oxidation of their ferro-magnesian constituents. The 

 Upper Amacura runs almost entirely through a gently undulating 

 granite and gneiss country, whilst its affluent, the Polvo de Oro, on a 

 tributary of which a man named Jeffrey is digging gold, traverses more 

 hilly ground composed of diabase and hornblende-schist — dykes in the 

 granite and gneiss. At its head the Polvo de Oro flows over a mass of 

 hornblende-schist dipping at a high angle with the horizon. Two 

 miles below Middle Camp a very fine exposure of quartz, eight feet wide, 

 is seen in hornblende-schist on the British side of the stream. I did 

 not procure any specimen of it, but have brought down pieces of gneiss, 

 granite, and hornblende-schist from Juanita, Middle Camp, and Polvo 

 de Oro heads. 



" As a gold district I do not consider there is a promising outlook 

 on the English side of the river, and though gold undoubtedly exists in 

 the Polvo de Oro and its branches, it does not seem to me to exist in 

 sufficient quantities to attract a large number of digsjers." 



The specimens from the Amacura, collected by Mr. Perkins, show 

 that, as he states, the country is gneiss and gneissose granite, the rocks 

 having the following characters : — 



Granitite-yneiss . — This is made up of a granitic aggregate of feldspai', 

 in part orthoclase and in part oligoclase, and of irregular patches of 



