Thfi J\)f(tr() ((ltd Kiiii'ihroiKj Rivers. 159 



half a mile west of Turaatumari are small rapids caused l)y a dyke of 

 diabase running parallel to the main one at the falls, whilst diabase is 

 exjiosed at intervals for about one mile west of the rapids. 



About two miles west of the cataracts the river traverses a broad 

 mass of fine-grained epidiorite, and exposures of similar rock occur at 

 intervals for about six hundred yards, the general trend of the rocks 

 being north-west and south-east. During the next two miles diabase is 

 seen at intervals, apparently forming portions of dykes trending north- 

 east and south-west. The low-lying banks between the mouth of the 

 river and about six miles west of Tumatumari consist largely of alluvial 

 and fluviatile deposits. About five miles west of Tumatumari the 

 river crosses a belt of schistose quartz-porphyry, first seen on the 

 "south bank and later trending across the I'iver in a northerly direction. 

 Near D'Amil's Stream the quartz-porphyry is more massive and has a 

 well-marked rectangular jointing ; exposures of this rock occur for 

 about three hundred yards, having a general trend of north and 

 south. One mile west of these exposures, the end of a great dyke of 

 diabase, of very coarse texture, juts into the river on its north bank, 

 and is there some sixty yards across. Its strike is north-west and 

 south-east, and it is succeeded by a fine-grained, greenish epidiorite. 

 West of this, for about a quarter of a mile to Garraway's Landing, a 

 belt of quartz-porjihyry is visible, the rock being usually more or less 

 schistose in character, but in places, as near the Konawak Creek Mouth, 

 it is massive. At Garraway's Landing quartz-porphyry is traversed by 

 a dyke of diabase of medium texture, trending north-north-east and 

 south-south-west, and exposed on the west bank of the river for about 

 six hundred yards, the river running parallel with the dyke. On 

 the south-eastern side of the river near Garraway's Landing, when the 

 water is low, the diabase may be clearly seen in contact with the 

 schistose ([uartz-porph3'ry, the coarse diabase at the contact showing a 

 chilled edge of fine-grained, very coinpact rock. About one hundred 

 yards north-west of the landing the maiii mass of the diabase is 

 intersected by a later flow of fine-textured dialjase M'ith a bedded 

 appearance. Quartz-porphyry, l)oth compact aiid schistose, occurs at 

 intervals from the noi-th-west end of this diabase exposure to about a 

 mile north of Potaro Landing, and is traversed by a dyke of fine-grained 

 diabase, trending east and west about a quarter of a mile east 

 of the landing, whilst two others having a similar trend cut through 

 quartz-porphyry west of that place. 



Two miles north-west of Potaro Landing tlie Kuril irong River joins 

 the Potaro, flowing from the north and noith-east in its lower reaches, 

 and from the north-west and west in its upper course. On the north 

 bank of the Potaro, and <m the eist bank of the Kuribrong at its 

 mouth, there is a great mass of ([uartz, whilst schistose (juartz-porphyry 

 is exposed on the west side of the latter river. At about a (juarter of 

 a mile south-west uf the Kuribrong Mouth are some small islets 

 and rocks in the river for about a quarter of a mile along its course, 

 appirently trending north and south. These consist of reddish, 

 dark-bi'own, and pur])!e-coloured shales derived either from a felsitic 



