GEOLOGY. 201 



Gold has occasionally boon found in the streams flowing from 

 these mountains, also along the Caroni, the largest southern 

 afflu(;nt of the Orinoco, and along the Paraguay, a tributary of 

 the Caroni, no valuable gold veins or deposits have ever been 

 discovered. These rocks seem to conform to the general law ; 

 namely, to be barren of productive gold veins. The Essequil)o 

 hydrograpiiical basin is the true gold-bearing portion of tiie rocks 

 of Guiana. So far as known, the rocks of this basin are as fol- 

 lows : Gneiss on its northern rim (Imitaca Mountains); a few 

 leagues south are low ranges of quartz and porphyry, Santa 

 Cruz, Charapa, and Chagunemul Mountains. On their fl.mks 

 are seen hornblendic, silicious, and argillaceous slates. Gneiss 

 "vvith domes, or vast expansion of quartz veins succeeds. As we 

 progress southward these domes of quartz form a very striking 

 feature of the landscape. They are more abundant east of the 

 Caroni River and south of the Imitaca Mountains than any other 

 portion of the country visited. They are always in sight. One is 

 constantly winding around them or crossing some low portion of 

 tiiem. Sometimes their outcropping rocks remind one of a dis- 

 tant cemetery with its slabs and monuments of white marble. 

 The gneiss decomposes and then presents a mottled appearance, 

 red, purple, grayish, and white in color. Dikes of granite, or 

 more properly, syenite, appear at intervals. Apjjroaching tlie 

 valley of the Yuruary River — the northern affluent of the Esse- 

 quibo — bands of white and light-drab limestone are seen with 

 the gneiss, and near Guasipati a band of itacolumite appears. 



After crossing the Yuruary River, hills and low mountains of 

 metamorphosed or semi-crystalline hills rise a thousand or fifteen 

 hundred feet above the valley. 



These mountains trend N.N.E. or S.S.W. They are composed 

 of the following rocks : Brecciated schists, altered sandstones, 

 quartz, and porphyry, a local rock of the aluminous family known 

 as blue-stone, and talcose schists. The porphyry, in many in- 

 stances, is but a highly metamorphosed condition of the more 

 silicious portions of talcose rocks. Talc and bluestone is the 

 country rock of the gold veins of this portion of the Essequibo 

 basin. Beside the rocks already described, there lie between the 

 sources of the Yuruary and the Caroni a low range of hills run- 

 ning north and south which are composed of very black gneissoid 

 schists and more solid rock dissimilar to the grayish gneiss of the 

 Imitaca. Tliese are older in geological time than the Imitaca, for 

 the latter trend east and west and abut upon them, while these 

 trend north and south. 



In the Mocujiio valley gold is found under the following modes 

 or conditions : — 



1. In the sands and gravel beds of the streams of the valley. 



2. In paydirt beds on bed-rock in the alluvial of the valley, 

 and in the clays derived from the breaking down and decomposi- 

 tion of the countrv rock of the veins. 



3. In quartz veins under different conditions, as follows : «, m 

 pure while quartz in granules and nuggets; 6, in rusty and ochra- 

 ceous quartz invisible to the naked eye ; c, in thin bluish and gray- 



