490 J. Allan Thomson—Inclusions in Voleanie Rocks. 
naturally an exceedingly slow one and was accomplished at consider- 
able depth beneath the earth’s surface; the flow of quartzite can only 
take place at certain temperatures and pressures, and directly these 
fall below a certain point, solution and redeposition of the substance 
of the constituent grains cease, and the rock becomes the hard, brittle, 
intractable rock one is familiar with. 
If I am wrong in my explanation of the causes which led to the 
formation of the Tygerberg anticline, I am quite sure that m 
description of the effects produced is correct ; I visited the hill many 
years ago before I had studied the main ranges to the south, but could 
make nothing of the difficulties, although I had Sawyer’s map and 
sections’ with me and found them correct; it was only later that 
I began to see the nature of the fold after spending a considerable 
time at the place. 
Turning to the larger mass of Witteberg Beds to the south which 
form the range of hills fronting the Swartebergen, the beds are 
perfectly exposed in the many kloofs, while there is very little 
vegetation to obscure the details. One can trace each individual 
stratum, and the whole series is accounted for; there is no break in 
the succession whence the root of the supposed overfold could have 
originated. The same is true of the main range of the Swartebergen,, 
for it is just in this longitude that there is the magnificent gorge, 
called Meiring’s Poort, where the structure of the mountain strata is. 
so clearly revealed that there is no possibility of the existence of a 
great limb missing. Contrariwise, on either end of the Tygerberg the 
Dwyka and Ecca Beds are folded in a gentle anticline continuing that. 
of the quartzite. 
IIlI.—Inciusions 1n some Votcanic Rocks. 
By J. Attan Tomson, B.A., B.Sc., F.G.S. 
M* attention was first drawn to this subject by the study of 
a remarkable crystal breccia, occurrmg at Kakanui, New 
Zealand, of which a description is given below. The only similar 
phenomena I could read of in the Colonial libraries at my disposal 
were those of the diamond pipes at Kimberley, described by Professors 
Carvill-Lewis, T. G. Bonney, and Miss Raisin in this Magazine.’ 
Having since had the opportunity, thanks to the hospitality of 
Professor Lacroix, of studying in his laboratory at Paris, I found 
that the proper discussion of this subject comes under the study of the 
inclusions of volcanic rocks, of which he has given so admirable an 
account in his book ‘‘Les Enclaves des Roches volcaniques.”* At his 
recommendation I collected some inclusions from the well-known 
1 A. R. Sawyer, Report on the Geology and Mineral Resources of Prince Albert : 
Parliamentary Report, Cape Town, 1893. 
2 Numerous scattered references to crystal tuffs occur in British petrological 
literature, and the subject is well presented by Sir A. Geikie in ‘‘ The Ancient: 
Volcanoes of Great Britain,” especially vol. i, p. 62, and vol. u, p. 79, but the 
general paucity of information on this point in the standard textbooks is remarkable. 
3 A. Lacroix: ‘‘ Les Enclaves des Roches voleaniques’’; Macon, 1893. 
