130 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
was formed under quite the same conditions. He thinks (ibid., p. 153) 
that boulders and pebbles as well as some components of the matrix 
of the conglomerate reached their present positions by means of ice- 
bergs and drifting floes. 
The microscopic characteristics of the ancient boulder-clay are 
mentioned by Molengraaf and Green. The former states (p. 260) 
that the mud contains numerous small angular fragments of different 
rocks and minerals, chiefly quartz, and that the clastic structure is 
modified by recrystallization, which gives the rock a strong resem- 
blance to volcanic tuff or breccia. Green (p. 242) notes the markedly . 
angular and varied shape of the fragments. He states that they are 
nearly all limpid quartz containing fluid cavities; “the fractured 
nature of the quartz grains comes out beautifully under moderately 
high power, but often other parts of the edge in the same grain are 
smoothed and softened off as if by the action of some solvent.” The 
writer has examined a slide made from a specimen of the Dwyka 
conglomerate kindly placed at his disposal by Professor Davis. In 
addition to the characteristics already noted the following features 
were observed. Several minerals besides quartz were present, includ- 
ing among others microcline, plagioclase, garnet, and biotite. The 
feldspar fragments showed little or no kaolinization and the mica 
flakes were frayed at the ends. 
The glacial boulder-beds of the Dwyka Conglomerate are not, 
strictly speaking, conglomerates, according to the definition given 
above (page 107). They are really such rocks as would be formed 
by the consolidation of masses of boulder-clay or till of the last gla- 
cial epoch. The name “tillite,” suggested by Penck during a recent 
visit to the South African field is more definite and distinctive. 
An important and characteristic structure, since shown to be sec- 
ondary, is described by Green. He says (p. 242) that “in many 
places no bedding is discernible, but where the rock is exposed 
it has a bedded look, parallel bands making their appearance on 
the weathered face, differing from one another in color, and some 
weather faster than others. Slabs resembling rudely shaped tomb- 
stones stand in parallel rows along the hillside.” The thickness of 
the formation is 500-800 feet (C. D., White, p. 306). 
——:—Correlation; India, Australia, South A frica. It appears that 
similar beds of glacial boulder-clay have been found in India, Aus- 
tralia, and South Africa. The general belief that they were contem- 
poraneous is well summed up by Oldham. He says (a, p. 206), “In 
Africa, India and Australia alike, certain beds contain abundant 
