CHAPTER X. 



THE PETROGRAPHY OF THE INTRUSIVE DIABASE. 



Diabase (Dolerite) occurs very plentifully in British Guiana, forming 

 ranges of hills and mountains having a general trend fi'om north-east 

 to south-west, from which radiate numerous dykes, varying in breadth 

 from tongues of a few inches to broad dykes a hundred yards or more 

 across. In the sandstone districts it also occurs in the form of sills, 

 not unfrequently of great thickness, which may be traced across 

 country for many miles. C. B. Brown recognised three of these as 

 the main ones traversing the sandstone, and marked their outcrops, 

 over very extended areas, in his geological map of the colony. 



Where the diabase occurs as mountain-masses it is of coarse texture, 

 and approaches to the granitic structure of gabbro. But as far as 

 I have examined the rock it always has, to a greater or lesser extent, 

 the ophitic structure characteristic of a true diabase or dolerite, and it is, 

 in my opinion, advisable to use the term diabase for it in all its varieties. 

 In parts of the larger sills and broader dykes the rock also exhibits 

 a coarse texture, but not often to so marked an extent as in the 

 mountain masses. The outer parts of the dykes and sills usually have 

 a much closer texture than have the inner ; and in places show very 

 compact chilled edges. In a few places the masses of diabase are 

 traversed by later dykes of the same rock, some of which possess a 

 columnar structure. In these secondary dykes the rock shows a 

 structure somewhat different from its usual one, the feldspar-laths 

 lying in a ground-mass of augite granules with scattered minute grains 

 of titaniferous iron-ore and magnetite, and with some interstitial 

 undifferentiated glassy matter, instead of in large irregular ophitic 

 masses of augite. In some of these rocks porphyrinic crystals of augite 

 or of labradorite have developed, and they might be termed augite- 

 porphyrite. The majority of the narrow columnar dykes or tongues 

 are best described as basalts of the tholeite type. 



In colour the diabase varies from dark -green to greenish-grey, grey, 

 and dark -grey ; it is most usually of a very dark stone-grey colour. 

 It is heavy, and, as a rule, dense in structure. Its clean-fractured 

 surface is dull and almost without lustre. The rock is hard, very 

 tough, and resistant to the hammer ; and blocks of it, when struck, 

 resound with a clear ringing tone. The weathered surface of the 

 masses of the rock exposed in cataracts or rapids in the courses of the 

 i-ivers are covered with a thin coating or skin of varying shades of 

 brown I'esulting from the oxidation of the iron-bearing minerals ; and 

 where its surfaces are not exposed to the scour of rapidly running 



