THE COXSTITLTION OF CERTAIN ROCKS OF THE ARCH-EAX AGE. 25c. 



be found. The felspars and quartz contain numerous crystal 

 inclusions of zircons and apatite. A marked similarity is thus 

 apparent between this type of rock and the rocks of the older 

 type of intrusions mentioned above. Now for a glance at the 

 rucks occurring- in that portion of the belt that lies between the 

 central zone of gneiss and its western limits. These too are \ ery 

 complex in character. Unfortunately the line of junction between, 

 them and the gneiss is not visible so far as I have seen, and thus 

 much information is withheld from view. Outcrops occur that 

 present to view rocks of a crystalline granular nature, whose com- 

 ponents have attained a large size and exhibit a speckled aspect 

 produced from the intermixture of greyish and greenish or reddish 

 and greenish or greenish and blackish coloured constituents. So' 

 great is the diversity in colours of one and the same or of the 

 different minerals. Other outcrops may exhibit a similarly coarsely 

 crystalline rock with large well-formed crystals of felspar standing 

 out vvithin the crystalline matrix and illustrating well the porphy- 

 ritic structure. I have seen rock of a similar nature jutting out 

 from the sea near Camps Bay, Cape Town. Again intermingled' 

 with these or either of them or associated with them is to be 

 found a type of a dark basic-like rock. The basic rock is so inti- 

 mately connected with the associated rocks and is usually of a' 

 finely crystalline nature that from the features in the field obtained 

 from an inspection of the limited outcrops it would be difficult tc 

 render any opinion as to its nature. In the so-called porphyries^ 

 the felspar is set in some cases in a quartz matrix which from the 

 marked parallelism of the longer axes of the felspars and the 

 homogenous nature of the quartz cement is indicative of the 

 former fluidity and flow direction of the ancient acid magma. At 

 places some of the constituents have disappeared from the quartz 

 cement through decay and the remaining cavities give the matrix 

 a pitted or porous appearance. \'ery often the so-called speckled 

 rocks weather into huge blocks along divisional planes which may 

 mislead anyone looking at them from a distance into the belief 

 that they marked the site of an outcrop of sandstone. The sudden 

 change that takes place in the nature and colour of the^oil and 

 at times of the vegetation and grass too within these confined 

 areas marks the change in the underlying rocks. This is borne 

 out by the examination of the soil in the neighbourhood of out- 

 crops. These rocks are overlain unconformably bv rocks of dif- 

 ferent formations ranging from the Table Mountain Series up- 

 wards. The difference in the colour of the so-called speckled' 

 rocks is due entirely to the wide range in colour assumed by the 

 felspars. Such are' the changes which occur and recur in the 

 western section of the belt. Microscopically these somewhat 

 coarsely crystalline speckled rocks are found to have had a former 

 porphyi-itic' structure. They are holocrystalline in character, and 

 the rock consists of an admixture of felspar, quartz, and horn- 

 blende as essential minerals, with several accessory minerals. The 

 felspar is the most abundant of all the constituents. Then quartz 

 but they possess the same mineralogical characters and structures. 

 Besides, thev occupv a minor part in the constitution of the rock 

 compared with the base. The hornblende, which is confined to 



