The Lower Essequibo River and the Cuyuni River District. 117 



not far from Patientia, and parts of the C(jal may easily have been 

 washed from her. Seams of coal nowhere exist in gneissose and 

 granitic rocks such as form the country in this part of the Essequibo. 



There are few exposui'es of rock on the right bank between Ithaka 

 and Ampa. The rociy islets, known as Pigeon Islands, consist of 

 granitoidal gneiss and of syenite, the western one, from which rock has 

 evidently been removed by blasting, bemg a medium-textured grey 

 granitoidal gneiss intersected by veins of pegmatite, while the eastern 

 one is a mass of a somewhat coarse-textured basic hornblende-syenite. 



At Ampa, five and a half miles south of Ithaka, a considerable area of 

 gneiss, veined with pegmatite, has been opened up by quarrying. The 

 gneiss is almost identical in character with the rock now being worked 

 in the two large quarries at Dalli, about three-quarters of a mile 

 south of Ampa. In these, the coarse-textured grey gneiss is 

 seen to be traversed in various directions by numerous intersecting 

 veins of pink-coloured pegmatite, containing large crystals of microline- 

 feldspar, blebs and masses of quartz, and plates of muscovite-mica, some 

 of which are one and a half inches across. The pegmatite- veins vary from 

 an inch to perhaps two feet in breadth. Near the top of the northern 

 quarry the gneiss is traversed by a vein of granite, varying from two 

 to three feet in breadth, the rock being rather fine-grained and closely 

 resembling the grey granite exposed at the Penal Settlement on the 

 Mazaruni River. In some of the veins of pegmatite small dark-i*ed 

 garnets are abundant. The Dalli Islands consist of similar gneiss. 



On the left bank of the river, between Patientia and Wolgar, a 

 distance of a little over four miles, exposures of gneiss occur at 

 intervals. The Wolgar quarries furnish instructive sections of the 

 gneiss. Here the rock is of two varieties, a light-coloured feldspathic 

 gneiss, and an almost black biotite-gneiss, approaching in character to 

 a biotite-schist, and having a specific gravity of 3 '00. Pegmatite- veins 

 traverse both the dark-coloured and the light-coloured gneiss, the rock 

 of the veins being as a rule white or pinkish-white in colour. In some 

 of the veins crystals of orthoclase occur six inches in length and 

 having well-defined faces. In places the pegmatite contains plates 

 of dark mica, some of which are two inches in breadth. In the 

 section shown in this quarry the biotite has separated from the bulk of 

 the gneiss into masses having the high specific gravity noticed above. 

 Having regard to the })roximity of the great mass of granite which 

 extends from near Wolgar to Akaio in the (Juyuni, a distance as the 

 crow flies of thirteen miles, and for a similar distance to the southern 

 extremity of Karia Island in the Mazaruni. It is probable that these 

 biotite-masses are of secondary origin, and have been derived from 

 rocks which are originally hornblendic. In places the veins of pegma- 

 tite gradually change by increase in the proportion of quartz and by 

 decrease in that of feldspar into quartz-veins. 



The islands known as Three Sisters in the river about three-quarters 

 of a mile in a southerly direction from Wolgar, consist of medium to 

 fine grained grey granite. For about a mile north of Makauria Creek, 

 the rocks exposed on the right bank consist of granite, and this is well 



