Tlip Mazaruni River. 139 



hundred yards of which appears to be traversed by the river which 

 here flows from the south. 



Above Kusawe Rapids the river winds through a gneissose country 

 for a Jong way, the length of this part of its course being about 

 twenty-five miles. Above Kusawe Rapids the gneiss is traversed a little 

 above the rapids by a narrow dyke of diabase, changed locally to protero- 

 base ; and about three-quarters of a mile from the rapids a dyke of diabase 

 of medium texture is exposed for about one hundred and fifty yards, 

 striking in a north-east and south-west direction through the gneiss. 

 At the lower end of the channel called Karawarambo Still- water, a 

 dyke of diabase strikes through the gneiss in a direction of about 

 north 10° west and south 10° east. Several broad exposures of 

 the gneiss occur in Karawarambo Rapids and channels and in 

 Pakaburi Rapids, Hole and Channel. At the head of Pakaburi 

 Channel, near thi^ foot of Little Itaki Rapids, a dyke of diabase about 

 five feet in breadth traverses the gneiss, striking north and south. In 

 this neighbourhood the striking differences in the manner of weathering 

 of gneiss of different structures is well marked — the coarser textured 

 parts altering into great rounded knobs or bosses, the finer ones 

 forming low hummocks, in places, with very rough surfaces resulting 

 from the partial weathering of the rock and the re-cementation of the 

 debris. 



Near Little Itaki Rapids (Itaki-boy Rapids) coarse- textured 

 hornblende-granitite-gneiss shows in places intrusive tongues of very 

 fine-grained, dark-coloured hornblende-granitite. At Itaki Cataracts, 

 about a mile above Itaki-boy, is a very broad exposure of gneiss, the 

 rock generally being granitite-gneiss, but in places being medium to 

 coarse-textured hornblende-granitite-gneiss, with liere and there almost 

 a granitoidai appearance. The gneiss is traversed in places by narrow 

 veins of granite and of aplite. About a mile above the head of these 

 cataracts the gneiss is traverse! by two narrow dykes of diabase, each 

 from ten to twelve feet across, and with parallel trends to the north- 

 east and south-west. Near the mouth of the Puruni atKumbiri Rapids 

 a dyke of diabase about thirty feet in breadth is seen for a distance of 

 about two hundred and fifty yards, striking north-north-west and south- 

 south-east through gneiss. The edges of this dyke are very compact in 

 structure ; and the joints in places are coated with films of arsenical 

 pyrite. An oft"-set from this dyke about five feet in breadth strikes to 

 the east-north-east near Flat Rock Island, and at Bird Island Rapids, 

 just below the mouth of the Puruni, a dyke of diabase trends through 

 gneiss in the direction of north-east and south-west, apparently 

 varying in breadth from twelve to (.'ighteen feet. 



Between the Puruni and Kurabiri Cataracts the channel passes 

 between many large hummocks of gneiss, which rock is traversed at 

 Kurabiri by a dyke of dark-coloured, medium-textured diabase about 

 twenty feet in breadth and trending north and south. Two off-sets 

 from the main dyke strike parallel to one another to the east and 

 west, one varying from six inches to four feet in breadth, the other 

 from three inches to one foot, the rock of these veins being of very 



