Tim Petrography of the, Intrusive Diabase. 87 



waters they are frequently covered with very rough layers, from one- 

 half to two or three inches in depth, of concretionary ironstone. The 

 coarse-textured varieties weather with a very rough surface, occasioned 

 by the masses of ophitic augite with their contained crystals of 

 feldspar being far more resistant to the action of rainwater than are 

 the intervening patches of lime-soda feldspar, which weather away, 

 leaving the former standing out from the surface of the rock. 



Not unfrequently by the sides of the rivers and streams, in 

 ravines, and in places in the heart of the forest, masses of this rock are 

 seen showing deep rounded flutings on their sides. These are very 

 characteristic of the diabase, but it is difficult to explain their origin, 

 the most feasible explanation as yet adduced being that they have 

 been produced by the drip through ages of rainwater from the branches 

 of overhanging trees. At some cataracts, as, for instance, at Turesi on 

 the Mazaruni River, where broad areas of diabase are exposed at certain 

 periods to the full glare of the tropical sun, both the horizontal and the 

 vertical surfaces of great masses of diabases are pitted with large and 

 small basin-shaped depressions. These are probably caused by the rock 

 flaking away by the expansion produced on its surfaces where exposed 

 to the rays of the sun, which frequently causes the dark-coloured rock 

 to become too hot to allow the hand to touch it with impunity, and by 

 the rapid contraction at night, when the temperature of the surface 

 becomes very quickly reduced below that of the mass of the rock by 

 radiation. The spheroidal structure so frequently present in great 

 masses of relatively fine-grained diabase causes the flakings, instead of 

 taking place evenly from the whole exposed surface of the rock, to result 

 in the formation of the pitted depressions. 



The products of the weathering of the diabase will be described in 

 the succeeding section. 



Examination of the surfaces of the coarser varieties of the diabase 

 by the unaided eye shows crystalline, irregular areas of light-brown 

 augite, interspersed with others of confused glassy-looking white laths 

 of feldspar, with, in places, black specks of iron-ore. In the finer- 

 textured kinds, the grey colour is seen to be due to the specking of 

 small irregular masses of brownish augite with minute flecks or tiny 

 rods of white feldspar, the latter becoming more noticeable as the 

 texture of the rock becomes coarser. Specimens from a few localities 

 in the colony show, in addition to the augite and feldspar, small oilv- 

 looking spots of a greenish-yellow tinge ; these consist of olivine, and 

 the rock is an olivine-diabase. Specimens of this may be found in the 

 more basic parts of some of the dykes of normal diabase ; but, as far as 

 ray experience goes, the occurrence of olivine in any quantity in the 

 diabase of British Guiana is far from common. 



As a general rule the diabase does not afiect the indications of the 

 compass ; it is not magnetic, its contained iron-ore being generally 

 ilmenite. But in places it does affect the compass readings, and it there 

 contains a good deal of more or less titaniferous magnetite. 



Examination of thin sections under the microscope shows that the 

 mineral components of the diabase are few. The chief mass of the rock 



