BUBIDGE — SOrTH-AEEICAlS' EOCKS. 



55 



subject is still somewhat defective ; I will defer what I have to say on 

 this subject till a future period. 



I have had but little time or opportunity for the microscopical 

 studies which have done so much for the view^s on the nature and 

 origin of granite which I am here advocating. I should hardly have 

 ventured indeed to have given observations so crude as my own, but 

 for a conviction that probably no country in the world offers greater 

 facilities for studies of this kind than does this Colony, and more 

 especially the district of Namaqualand, which is probably barer of 

 vegetation and more intersected by gullies than any other country in 

 the world not absolutely uninhabitable. 



I will give a hrie^ resume of the observations which led me first to 

 doubt and at length to abandon the igneous theory of granite, in 

 which I was a firm believer ere I visited the "Western Province of the 

 Colony. 



1. The undoubted change which rocks have undergone into quartz- 

 ite and its equally evident origin in superficial and igneous agency. 

 Mr. Darwin admits this origin of the Table Mountain sandstone. 



2.. The existence of beds of granite and other rocks of felspathic 

 bases in association with sedimentary rocks in positions which it is 

 impossible to believe they could have occupied by forcible intrusion 

 from below. Many veins of the clay stone-porphyry exceed a thousand 

 yards in width, yet they do not in the slightest degree disturb the 

 strata adjacent to them. At Kleinpoort I measured the slate eighteen 

 inches from its junction with the porphyry. It dipped towards the 

 latter at an angle of 35°, the porphyry itself having a dip in the 

 same direction. 



3. The irregular masses of granite taking the place of gneiss and 

 not connected with the granite below. 



4. The origin of prehnite and other zeolitic minerals from decom- 

 position of igneous dykes of the Dicynodon-strata. Prehnite, as 

 well as of quartz, is formed thus between the decomposing " boul- 

 ders " of igneous rocks. Veins of carbonate of lime are often formed 

 in the same way. JN^or can I hesitate to refer the felspathic veins 

 and irregular masses in decomposing gneiss in Namaqualand to a 

 like process of re-arrangement. I have there seen carbonate of lime 

 in felspathic rocks ; fluor-spar mixed with epidote and felspar ; phos- 

 phate of lime with felspar and quartz. 



5. I have mentioned the igneous dykes of the Dicynodon-strata. 

 They have always been referred to plutonic agency, but it appears to 

 me that there are great difficulties in admitting such origin. They 

 take, I believe, every direction of the compass, vary from eighteen 

 inches to some hundreds, perhaps thousands, of yards in breadth, and 

 some of them are probably fifty or more miles in length ; they are 

 numerous, and occur frequently from near Somerset East to the Vool 

 Eiver, but never, in my experience, or that of any one I know, pass 

 the boundary of the Dicynodon-strata, nor do they disturb the rocks 

 through which they cut in the base. 



6. I have mentioned the occurrence of granite-veins conformable 



