54 



THE GEOLOGIST. 



The sections of the Zeurberg, of Yan Zonder's Plain (given above) 

 on the Graff Eeinet road, together with a somewhat imperfect re- 

 collection of that of Graham's Town, had led me to believe that the 

 relation of the porphyry to the slate was constant. It is now evident 

 that I was in error here. The fact of the masses of the porphyry 

 crossing the slate without disturbing it seems to me greatly to 

 strengthen my views as to its metamorphic origin by rendering the hy- 

 pothesis of Messrs. Bain and Wylie untenable. The former gentleman 

 supposed that the porphyry had been poured over the surface of the 

 strata as liquid lava. Mr. Wylie referred its origin to volcanic ac- 

 tion, producing ash, which was deposited at the bottom of the ocean, 

 and formed this igneous-like rock with its contained granite pebbles 

 and fragments of rock. The fact of the direction of its masses being 

 at an angle of 30° with the strike seems to me to be incompatible with 

 either of these hypotheses. The ranges of porphyry, like those of 

 quartzite, die out and reappear. The normal position of the porphyry 

 appears to be as in fig. 4, but I have seen it placed as in figs. 5 and 

 6:— 



Slate. Porphyry. Slate. Slate. Porpliyry. Slate. Slate. Porphyry. Slate. 



The character by which we all agree to recognize this rock is the 

 presence of masses of quartz and granite of various sizes with occa- 

 sional fragments of slate and other rocks. Sometimes these masses 

 are as much as fifty pounds in weight, at others they are so minute 

 as to be scarcely recognizable by the naked eye. In a recent journey 

 to Paardenpoort I met with a mass of this porphyry whicli terminated 

 in a vein about a foot thick, with very minute crystals. jNTow what 

 is the character of the rock among which this porpliyry is interposed 

 or interstratified P It is such that no one acquainted with the two 

 would pretend to diagnose them, save by the presence of the crystals 

 above mentioned. Nor would the blowpipe, or even more careful 

 analysis, so far as I am aware, enable him to do so. If then the base 

 of the rocks difter so little, and there is evidence that no displacement 

 has taken place in any known section (see Bain and/\Vylie), is it not 

 clear that this rock has originated in slow conversion ? Yet I be- 

 lieve whatever may be predicated of it may be equally so of granite ; 

 for it contains granitic masses in great numbers, and often of large 

 size : besides granite, veins occupy precisely the same position among 

 rocks which I have given reasons for believing to be the same strata 

 in the AYesterii Province. 



But it will be seen above that I am not disposed to admit that the 

 evident (lis]ilacomont of strata is at all times due to eruptive agency. 

 I have given in^^lnnces, on a very small scale, in wliich 1 feel sure it is 

 not so. 1 hope ere long to be able to show that the infiltration of 

 quartz from above has produced this effect, but my evidence on this 



