SCHOMBTJRGK ON THE GEOLOGY OF GUIANA. 299 



boring through the clay to these sands, and in such cases the depth 

 from the surface to which it has been necessary to sink varies from 

 120 to 230 feet.* 



The clay extends for a considerable distance inland, and is 

 then terminated by a range of sand hills from 30 to 120 feet high, 

 parallel to which may be traced a number of detached groups 

 of hillocks, seldom more than 200 feet high, and consisting of 

 red iron ochre with occasional layers of silicate of zinc. This is 

 succeeded by a quartzose sandstone resembling the flexible sand- 

 stone of Brazil. 



The first unstratified rocks occur near Itaka, and consist of dif- 

 ferent varieties of granite with numerous greenstone dykes, and 

 of porphyry, while in the surrounding mountains, at no great dis- 

 tance, clay iron-stone was observed in small detached nodules. 

 This rock is repeated again towards the south, and occupies the 

 extensive plains or districts of table land, called " Savannahs," 

 which are about 300 feet above the sea, and from the midst of 

 which rise isolated hilly groups from 1800 to 2300 feet above that 

 level. These plains are covered by a conglomerate, containing 

 rounded fragments of quartz, and vast quantities of bog iron ore, 

 while blocks of granite, some of them of large size and much 

 rounded, also abound. The hills are porphyritic, and exhibit also a 

 considerable quantity of mica in foliated masses. 



A sandstone resting upon the small-grained gneiss and coarse 

 granite of the Savannahs is next described by the author as form- 

 ing the mountains of Pacaraima which extend from the upper 

 Orinoko, eastward, to the banks of the river Essequibo. Towards 

 the north, felspar-porphyry, and jasper, are also mentioned, and 

 then succeed lofty escarpments of sandstone rising almost per- 

 pendicularly from the plain, and forming the commencement of an 

 extensive range of high table land. This sandstone is described as 

 entirely destitute of organic remains. 



Having alluded to these rocks, the author next describes some 

 of the appearances presented by the clay, and other materials in 

 the bed of the river Cukenam, near which on both sides rise loftv 

 mountains, on whose declivities nodules and large blocks of 

 pisiform bog iron ore are found. After this follows an account of 

 another region on the right or western bank of the Cukenam, in 

 which jasper is so abundant as to form the prevailing rock. The 

 mountains are described as rising in a highly picturesque and 

 striking manner to the north of this locality, and are said to be 

 composed of compact sandstone, whence it appears that this rock 

 occupies the highest summits from the banks of the Orinoco to- 

 wards the south-west, and a similar ridge has also been traced by 

 the author considerably to the west and south. Large blocks of 

 granite are also mentioned by the author as abounding on the 

 flanks of the highest mountains, one of which " Roraima " is espe- 



* Ten or twelve feet below the upper surface an irregular stratum of fallen 

 trees (Avicennia nitida of botanists) is met with, and a similar bed, 12 feet 

 thick, has also been found at a depth of 50 feet. 



