42 The Geology of the. Gold Fiehh of British Guimia. 



The gneisses may be classified as follows : — 



Granite-gneiss . . North- Western district . . Vulcanose. 



,, ,, . . Cuyuni River . . Yellowstonose. 



,, ,, . . Mazaruni River . . Lassenose. 



,, ,, . . Essequibo River . . Yellowstonose. 



Honiblende-granitite-gneiss, Cuyuni River, Toscanose. 



,, ,, ,, Mazaruni River, Pantellerose. 



,, ,, ,, Essequibo River, Bandose. 



Diorite-gneiss . . Mazaruni River . . Andose. 



The Epidiorite, Amphiholite and Hornblende-schist Group. — The 

 gneiss of the colony is traversed by a series of rocks originally basic 

 intrusives and now epidiorites, amphibolites and hornblende-schists, 

 which form well-marked features in the topography of the gneissose 

 districts, giving rise to ranges of hills and to isolated knolls easily 

 distinguished from the suri'ounding gneissose j^enoplains. In the river 

 courses the basic rocks are frequently the cause of small cataracts and 

 rapids. 



The rocks of the group fall into two classes according to the intensity 

 of the dynamo-metamorphic forces which have affected them ; those less 

 affected, the epidioi'ites and amphibolites, in some places retaining as 

 kernels to plates of hornblende portions of the pyroxenes of the 

 original intrusive rocks which they now represent, whilst in the other 

 — the hornblende-schists — no traces of the original ferro-magnesian 

 minerals are seen. In many parts of the country it is not easy to 

 connect the well-foliated hornblende-schists with the massive epidiorites 

 and amphibolites, but in the Blue Mountain disti-ict and in the 

 Mazaruni-Puruni district, where these rocks occupy very large areas, 

 the transitions from the massive rocks through actinolitic ones to true 

 hornblende-schists can be traced both in the field and in thin slices 

 under the microscope, while at one place on the Cuyuni River I found 

 a series of rocks showing the transition from a gabbro to a hornblende- 

 schist. In the following accounts the rocks are arranged in inverse 

 order to the degree of metamorphism to which they have been 

 subjected. 



In the belt of basic rocks which cross the Mariwa or north-eastern 

 channel of the Cuyuni River at and near the Upper Mariwa Rapids, 

 from one of the spurs of the Blue Mountain hills a change from 

 gabbro to amphibolite and thence to hornblende-schist can be followed, 

 and this shows that the basic intrusives of the district were rocks of 

 the gabbro-diabase type. The following varieties of rock are found 

 at these rapids : — 



Below the rapids is a purplish rock of specific gravity 2*84, which 

 consists of a granitic aggregate of labradorite, some microcline, and 

 some interstitial ({uartz, plates of almost colourless augite, with some 

 peripheral chlorite and serpentine, many of the augite plates with 



