Tlip. Potrogrnjjhy of thfi Intrumvp. Diabcm'. 93 



with a little enstatite, and with some peripheral green hornblende 

 and a little chlorite ; some large irregular grains of titauiferous iron-ore 

 and a few minute prisms of apatite. Patches of zeolite and some plates 

 of carbonates are present as alteration-products. 



In the augite-granophyres, the best example of which was found 

 in the upper part of the Konawaruk River, a micro-pegmatitic ground- 

 mass of feldspar and quartz contains crystals of labradorite, which in 

 places are more or less clouded, patches of quartz, irregular granules of 

 a pale, almost colourless, augite, and some granules of titaniferous iron-ore. 



Banaltic-diabasc or TJioleite. — This type occurs only in narrow 

 dykes ti^aversing some of the larger masses of the normal rock, as at 

 Tumatumari, and in the chilled margins of some of the smaller diabase 

 dykes. It is a dark-coloured, fine-grained rock of specific gravity 

 ranging from 2-95 to 2-99. The rock consists of small prisms of 

 labradorite, their edges in places showing signs of corrosion and some 

 small granules of augite ; embedded in a ground-mass of augite, minute 

 grains of magnetite and some ill-defined interstitial matter, largely 

 consisting of glass. 



TacJbijUte. — A dyke near Kuiai Lake, on the Berbice River, consists 

 of a very fine-grained diabase, of specific gravity 2-98, and has a glassy 

 tachylitic margin of specific gravity 3-01. This margin consists of a 

 crypto-crystalline glassy matrix containing very abundant minute 

 grains of magnetite, and traversed by cracks which are filled with 

 chlorite and calcite. In places flow-structure is well marked. Few 

 diabase dykes in British Guiana show margins of this sort ; and this is 

 the sole instance that has been found during my recent investio-ations ; 

 but other instances, from places south of where these examinations have 

 extended to, have been brought to my notice. 



On the edges of some of the larger dykes the diabase is a good deal 

 altered, the feldspars being saussuritised to a greater or a less extent ; 

 and the outer parts of the augite-masses changed to pale-blue or f'reen 

 hornblende in places, to such an extent that the former is almost or 

 even entirely replaced by the latter, while in others chlorite has been 

 developed at the expense of some of the ferro-magnesian minerals. 



As a rule, these contact-rocks contain more }3yrite than does the mass 

 of the diabase; and, as this is generally more or less auriferous, in 

 many cases the former contain far higher proportion of gold than do 

 the latter. This was the case in samples I collected in the Essequibo 

 district, the modified diabase yielding 26 grains of gold to the ton of 

 the rock, while the diabase gave only at the rate of 5 grains. In a 

 few places, as at the Growler mine in the Potaro gold field, contact- 

 rocks are in parts rich in gold, some samples having yielded upon 

 assaying at the rate of 5 ounces of gold to the ton of the rock. These 

 exceptional instances were excluded from the foregoing calculation. 



In parts wdiere diabase has sent very long and narrow tongues into 

 acidic rocks, such as granite or gneiss, the basic rock, in addition to 

 undergoing the endomorphic changes which have resulted in altering 

 the augite to hornblende or to chlorite and epidote, has taken up some 



