142 The Geology i)f the Gold Fields of British Guiana. 



strikes north 10" e;i>st and south 10° west, in the epidiorite, near the 

 diabase dyke ; and about twenty-five feet in length with a breadth of 

 about two feet of it is visible. Samples taken from it yielded, 

 when assayed, at the rate of fifteen grains of gold to the ton of 

 rock. Near the left bank of the river are great masses of glassy, 

 white quartz, with somewhat contorted structure, apparently 

 parts of a vein striking north-west and south-east, whilst near by is 

 another reef trending north-east and south-west, both being in very 

 coarse-textured (juartz-diorite. Samples from these veins yielded at the 

 rate of twenty-six grains of gold to the ton of the quartz. A little above 

 these latter exposures of quartz a naxrow dyke of diabase trends east 

 and west through the quartz-diorite. During the next mile of the 

 course of the river many exposures of the latter rock occur, in one 

 place being traversed by a dyke of fine-grained diabase largely altered 

 to proterobase, the dyke being about ten feet in breadth and trending 

 south-east and north-west. Near it an intrusive vein, about eight 

 inches in breadth, of a micrograuite, having a fluxion structure, 

 traverses the c[uartz-dioi"ite. A quarter of a mile from here a diabase- 

 dyke strikes east and west through epidiorite, near where are some 

 small exposures of quartz ; beyond this another narrow dyke of diabase 

 strikes north and south. Exposures of fine and of medium-grained 

 epidiorite occur, followed by rounded masses of coarse epidiorite, with 

 an intrusive tongue of diabase through it from a dyke trending north- 

 west and south-east. 



About half a mile further up the river the epidiorite is succeeded 

 by a band of a hornblende-gneiss containing many blebs of bluish, 

 opalescent quartz. For about two miles the epidiorite has a more or 

 less gneissose structure, and in one place it is intersected b}- a narrow 

 dyke of diabase, about four feet in thickness, with a strike of north 

 20" east and south 20° west. Near the dyke the epidiorite is generally 

 of compact texture, and has weathered into angular masses. About 

 four hundred yards beyond this dyke a narrow vein of quartz, trending 

 north 50° east and south 50° west, is seen in the epidiorite. Above here 

 the epidiorite is traversed by a dyke of diabase, about twenty feet in 

 breadth, striking north-east and south-west, and from this there is a 

 narrow offset of fine-grained diabase. Alternations of coarse-textured 

 epidiorite, more or less gneissose, and of a fine-grained, compact variety, 

 extend over the course of the river to near the end of the Tamanua 

 Channel. These are traversed in places by narrow dykes and 

 veins of fine-textured diabase, and towards the end of the Tamanua 

 Channel by two dykes of the same rock, each about eighteen feet in 

 thickness, striking, parallel to one another, in an easterly and westerly 

 direction. 



In the district extending from Turesi Cataracts to the Marabisi 

 Channel, the more feldspathic parts of the epidiorite or quartz-diorite 

 generally weather into rounded masses, and in places give rise to 

 somewhat extensive low rounded hummocks, while the more horn- 

 blendic and basic portions give rise to angular rocks, blocks and 

 boulders. 



