CLIMATES OF GEOLOGIC TIME. 295 



sandstone, but the contact is not exposed. The tiUite is overlain by a massive stratum of lime- 

 stone and of fissile layers, and then iDy thicker strata of limestone. No fossils were found in the 

 limestone or in the talus at the base of the cliffs to the west of Tungling. 



"On the south side of the river, opposite Tunghng, tillite is well exposed on the beach near 

 the water beneath limestone, with strike N. and S., and a dip 25° W. The tillite is well indurated, 

 and carries fewer boulders in its upper than in its lower portion. The erratics embrace granite, 

 gneiss, quartz-porphyry, slate, limestone, and quartz. The tillite is here about 250 feet thick, 

 and rests on red sandstone, the contact with which is seen for a short distance only. The under- 

 lying sandstone, about 100 feet thick, becomes more pebbly and conglomeratic toward its base, 

 and is arkose near its bottom. It rests on decomposed hornblende-schist, no granite being seen 

 here on the south side of the river. On the north side there is decomposed granite beneath the 

 sandstone, and farther off there is hornblende-schist. 



"From these observations it appears as though the tillite extended continuously from where it 

 is seen beneath the limestone east of Nantou, around the south flank of the granite-gneiss area, 

 to and beneath the limestone cliff west of Tungling, and that it extends northward from Nantou 

 at least some distance. It appears to be thickest south of Huang-Ling-Miao and thinnest in 

 its extension toward the northeast. 



"I spent parts of five days in five different places hunting for fossils without seeing one, so 

 they must be scarce. The rocks appear to be free from trace of them for some hundreds of feet 

 in thickness, as I examined carefully the talus below several high cUffs. I was disappointed not 

 to find any." 



From the fact that no one has yet found fossils in the very thick and well-exposed dolomites 

 that overlie the tillites in the immediate area of the glacial deposits, the question may well be 

 asked: How do we know that the dolomites at their base are of Middle Cambric age? In order 

 to answer this question, the writer corresponded with Blackwelder on the subject, receiving the 

 following reply under date of March 24, 1918. 



"In the Yangtze region there are two very thick massive limestone formations, neither of them 

 rich in fossils, but both with their own distinctive lithologic characters. These are separated by an 

 easily recognized body of green shale. In crossing the mountains north of the Yangtze River, and 

 in followng down the great gorges of the latter, these formations were repeated again and again in 

 great open anticlines and synclines, interrupted by only occasional faults. The age of the lower 

 massive limestone was approximately fixed by the finding of a rich Mohawkian fauna near the top 

 of it, perhaps 50 miles north of the Yangtze River. 



"The Middle Cambrian fossils which were found several days' journey northwest of the tillite 

 exposure, came from rocks very highly folded and faulted, so that their stratigraphic relations are 

 uncertain. Lithologically, however, the beds were much like those just below the great cliff lime- 

 stone at Nan-t'ou, in that they contained many layers of gray and black oolite. I think it highly 

 probable that the horizon of the Middle Cambrian fossils is not more than a few hundred feet above 

 the tillite and is beneath the great cUff limestone." 



On the ground that none of the Lower Cambric faunas indicate cool or cold waters and in 

 view of the further fact that in many widely separated ])laces there are reef-making corals 

 (Archaeocyathinse), the writer does not regard the Yangtze tillites as of Cambric age. He would, 

 rather, refer them to the Proterozoic, but whether they are late or early in this era remains to be 

 determined on the basis of the age of the superposed dolomites. 



EARLIEST PROTEROZOIC GLACIATION OF CANADA. 



Coleman says : 



"For several years it has seemed to me very probable that there was a still more ancient 

 ice age, at the beginning of the Lower Huronian in the Archean as defined in Canada, or the 

 Archeozoic or lowest Algonkian, as defined by various American geologists. The so-called 

 Huronian 'slate conglomerate' of Ontario has attracted attention ever since Logan and Murray 

 mapped and described it in the typical region north of Lake Huron, nearly fifty years ago. Good 

 descriptions of it are given by Logan in the 1863 report of the Canadian Geological Survey; 

 where he refers to the different kinds of rock enclosed as pebbles or bowlders, granite, felsite, 

 certain greenstones and jasper, for example; and describes the matrix a.s sometimes slaty, some- 

 times more quartzitic or like diorite or greenstone. At present the matrix would generally be 

 called graywacke or slate, though sometimes it is schistose or looks like an eruptive rock. 



