CLIMATES OF GEOLOGIC TIME. 269 



DEVONIC GLACIATION. 



In South Africa there occurs, beneath Lower Devonic marine strata, the 5,000-feet- 

 thick Table Mountain series, essentially of quartzites with zones of shales or slates, which 

 has striated pebbles up to 15 inches long, found in pockets and seemingly of glacial origin. 

 There are here no typical tillites and no striated undergrounds have so far been discov- 

 ered. While the evidence of the deposits appears to favor the conclusion that the Table 

 Mountain strata were laid down in cold waters with floating ice derived from glaciers, it 

 is as yet impossible to assign to these sediments a definite geologic age. They are certainly 

 not younger than the Lower Devonic, but it has not yet been established to what period 

 of the early Paleozoic they belong. 



Elsewhere than in South Africa, late Siluric or early Devonic tilhtes are unknown. 

 It is desirable here, however, to direct attention to the supposed tillites mentioned by 

 Ramsay and found in the north of England in the Upper Old Red Sandstone of late Devonic 

 time. Geikie (1903: 1001, 1011) states that this "subangular conglomerate or breccia 

 recalls some glacial deposits of modern time." Jukes-Brown in his book, "The Building of 

 the British Isles," 1911, writes of arid Devonic climates, but does not mention tillites nor 

 glacial climates. Fiu-ther details as to this and other pre-Permic glaciations are given 

 in the Supplementary Notes at the end of this chapter (pp. 290-296), chiefly in the form 

 of quotations from original sources. 



CAMBRIC GLACIATION. 



Unmistakable tillites, thought to be of earliest Cambric age, have been described by 

 Howchin and David from southern Australia and by Willis and Blackwelder from China. 

 In both cases the evidence as to age is open to question, as the tillites are either sharply 

 separated from the overlying Cambric deposits or these strata have no fossils to fL\ their 

 age, thus leading to the inference that the tillites are more probably of late Proterozoic 

 time. In Arctic Norway occur other tillites at the base of the thick Gaisa formation. 

 These deposits also were formerly regarded as of Paleozoic age, but Norwegian geologists 

 now refer them to the Proterozoic. All of these tillites are best referred to the vast era 

 previous to the Cambric period. 



LATEST PROTEROZOIC GLACIATION. 



Australia. — In southern Australia, conformably beneath marine and fossiliferous Lower 

 Cambric strata but sharply separated from them, occur tiUites of wide distribution. They 

 extend from 20 miles south of Adelaide to 440 miles north of the same city, with an east- 

 and-west spread of 200 miles. Boulder-clay has also been discovered on the west coast of 

 Tasmania. The tillites range in thickness from about 600 to 1,500 feet and occur at the top 

 of a vast pile of conglomerates, grits, feldspathic quartzites, slates, and phyUites, whose 

 exact age is unknown because as yet no fossils have been discovered in them. (See figure 89.) 



According to Howchin, the tillite consists "mainly of a ground-mass of unstratified, 

 indurated mudstone, more or less gritty, and carrying angular, subangular, and rounded 

 boulders (up to 11 feet in diameter), which are distributed confusedly through the mass. 

 It is, in every respect, a characteristic till" (1908: 239). The first scratched boulders were 

 observed in 1901 and now they are known by the "thousands" (David). They range in 

 size up to about 10 feet long. So far, no striated underground or glaciated floor has been 

 cUscovered, and both Howchin and David hold that the tillite was formed at or near sea- 

 level in fresh or brackish water with floating icebergs. The rocks of the tills, David thinks, 

 came from the south. The tilUte is now found from below sea-level to about 1,000 feet 

 above the sea. These tillites and all of the enormous mass of coarse deposits below them, 

 which is at least several miles thick, the Australian geologists regard as of Lower Cambric 

 age, because overlying them occur fossils of this time. The contact between the tillite 



