General Geology. 



there are well-marked local disturl)ances in its dip. Many of the beds 

 of sandstone of finer texture sh()\v well-marked current-bedding. 



Granite. — True granite with abundant orthoclase and both white and 

 black mica does not appear to be an abundant rock in British Guiana. 

 The largest development of it as far as I have been able to ascertain is 

 the great mass extending from Makauria Point on the Essequibo to the 

 south end of Karia Island on the Mazaruni, and to the foot of Akaio 

 Rapid on the Cuyuni. Siiniliar granites, but of somewhat gneissose 

 structure, also occur at Granite Island and at Canayaballi on the Waini 

 River, and on the Pomeroon River. There is also an exposure of a 

 granite on the Curiebrong River. These granites appear to be the 

 5'oungest of the basal igneous rocks of the colony, and from Brown's 

 observations at Akalikatabo Island, in the Courantyne River, may be 

 in parts of more recent origin than the sandstone formation. As far as 

 I have examined specimens of it the granite always shows either 

 uiacroscopically or microscopically signs of mechanical stress and of 

 incipient metamorphism, although this is but little marked in parts of 

 the great Essequibo-Cuyuni-Mazaruni mass. 



Belts of granite with only black mica, which may be in part replaced 

 by hornblende, and with relatively abundant plagioclase-feldspar. 

 are of fairly common occurrence in the colonv, traversing the })orphyry 

 and felsite rocks, the various schists, and tlie gneiss. The rocks of 

 these belts are granitites, hut jiarts of the great Mazaruni granite mass 

 above mentioned pass by imperceptible modifications into Ijiotite- 

 bearing rocks of this class. As a rule the effects of dynamic 

 metamorphism are more marked in the granitites than in the granites, 

 and many of the belts traxersing the country are clearly seen to be 

 gneissose granitites. 



From tl;e masses of granite and of granitite man}' dykes, veins and 

 tongues of aplite, of muscovite-granite, and of micro-granite penetrate 

 and intersect the adjacent rocks. The belts of granitic rocks, being 

 more resistant to weathering than are the porphyries, schists and 

 gneisses, usually give rise to low hill ranges or to isolated rounded hills, 

 and where rivers have cut their courses across their belts the occurrence 

 of the granitic rocks is usually marked l)y the presence of rapids or of 

 low cataracts. In addition to the granites and granitites, hornblende- 

 granite or quartz-diorite is not of unfrequent occurrence. in places 

 augite is the principal ferro-magnesian mineral present, the rock 

 becoming an augite-granitite ; in others, though rarely, hornblende- 

 granite passes into a true syenite, while true diorite seems to be a. rock 

 of rare occurrence in the district. Bands of augite-syenite also occur 

 in a few places in British Guiana. 



Quartz-porphyries, Porpliyritis, FeUitux, ForphyruiJs, and Sericife- 

 schi'^ts — Large areas of the colony are occupied V)y rocks belonging 

 to this group, the massive members of which gradually and almost 

 imperceptibly pass into the foliated schistose ones, and it is not 

 possible in many places to demarcate one variety from the othei'. 



