96 



The Gcoloijy of thfi (inld Fields of British Guiana. 



In addition to the constituents mentioned in the foregoing analyses, 

 careful examinations have shown that all the samples of diabase 

 collected have contained more or less gold and silver ; the former in 

 (juantities varying from unweighable traces to 17 grains per ton of the 

 rock, the latter from 3 to 5 gx^ains. 



The following analyses illustrate the composition of the altered 

 diabase containing tureen hornblende and chlorite : — 



Kssequibo and 

 Potaro Rivers. 



Mazaruni River. 



Silica 



Alumina ... 

 Iron peroxide 

 Iron protoxide ... 

 Magnesium oxide 

 Calcium oxide . . . 

 Hodium oxide 

 Potassium oxide... 



Water 



Carbonic anhydride 

 Titanium oxide . . . 

 Phosphoric anhydride 

 Chlorine ... 

 Iron sulphide 

 Cobalt oxide 

 Manganese oxide 

 Barium oxide 

 Copper oxide 

 liCad oxide 



50-72 

 16-35 

 4-07 

 8-06 

 6-81 

 7-08 

 2-38 

 0-66 

 2-87 

 1-50 

 0-12 

 0-001 

 trace. 

 0-005 



trace. 



trace. 



52-11) 

 15-47 

 8-30 

 3-65 

 4-59 

 8-26 

 2-71 

 1-48 

 0-90 

 0-15 

 1-24 

 0-10 

 0-13 

 0-26 



0-13 



0-08 



trace. 



99-(;2(; 



99-66 



THE MINERALOGICAL COMPOSITIONS AND CLASSIFICATION OF THE DIABASE. 



From the chemical and microscopical examinations outlined in 

 previous pages the probable mineralogical compositions of the diabase 

 rocks, and their classification in accordance with their ultimate 

 magmatic constitution, have been calculated. The mineralogical com- 

 positions, calculated from the results of the analyses, differ from those 

 shown by the microscopical examinations by the relatively high pro- 

 portions of uncombined silica indicated in them, and which should, 

 presumably, be present as quartz in the rocks. Although quartz is 

 shown by the microscope to be of rather common occurrence in these 

 diabases, its proportion in no case approaches that indicated by the 

 calculations. The feldspar present in them is shown by the analyses to 

 be a basic labradorite or bytownite. This is in accord with the optical 

 determinations. Tlie augite present appears to contain relatively large 



