275 



is a dark blue. This is in all probability Eiebeckite, which 

 has only lately been found in the granites of Socotra and some 

 of the British eurites. A notable point in conection with this 

 ■outcrop is the absence of dykes, which form such a characteris- 

 tic feature in all the others. 



At Corney Point the rocks consist of gneiss and hornblendic 

 schist, intersected in all directions by dykes of pegmatite 

 ■cha.racterised by large masses of microcline and oligoclase fel- 

 spars and segregations of quartz. That these are dykes and 

 not segregation veins is shown by the following facts: — 



1. That in some cases we find fragments of gneiss entangled 

 in the intrusive masses. 



2. That the dykes strike across the folia of the gneiss, which 

 in some cases have been drawn round from their original posi- 

 tions into a direction more or less parallel to the line of flow 

 ■of the intrusive mass in a manner suggestive of a semi-viscous 

 mass forcing its way through a fissure in a rock reduced to a 

 quasi-plastic condition by heat and pressure. 



3. Contact metamorphism is strongly evidenced in the 

 neighborhood of many of the dykes by the development of large 

 masses of biotite at the line of contact, and sometimes by an 

 alteration of the adjacent rock. 



In all other cases the metamorphic rocks are similar to those 

 of Corney Point, containing segregations more or less acid 

 in character, and sometimes crystals of wolfram, amphibole, 

 &c. On the south side of Daly Head, which, like most of the 

 others, is composed of Pleistocene sands resting on metamor- 

 phic rocks, they occur in the form of low reefs running out 

 into the sea, separated by short stretches of sand. As far 

 as can be seen, these reefs a*re identical in composition. A 

 curious point is that two adjacent reefs, like the pebbles of 

 MacDonnell Bay, are completely coated with silica, while in 

 the others this phenomenon is absent. Why these and not 

 the other reefs should be so affected it is difficult to see, as they 

 all strike in the same direction and occupy simila*r positions 

 relatively to the sea. 



In the gneiss at the base of Cape Spencer a large dyke of 

 •dolorite occurs, resembling a huge dumbbell in shape about 

 100 ft. long by 50 ft. in width at the broadest part. This 

 is intersected along its major axis by a dyke of gneissic aplite 

 which meets a narrow dyke of gneissic biotite oranite 

 running through the dolorite at right angles to the former. 

 'The granite and aplite dykes, however, do not cut one another. 



An outcrop of metamorphic rocks extends along the coast 



