Page:The Geologist, volume 5.djvu/73

Rh 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 views 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 brief résumé 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 quartzite 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 claystone-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 "boulders" of igneous rocks. Veins of carbonate of lime are often formed in the same way. Nor 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; phosphate 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 River, 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