SINIAN GLACIAL DEPOSIT. 39 



these the waters were clear. Being partly closed and shallow, they were 

 relatively warm and liable to maximum evaporation. Rippling of the 

 surface favored precipitation of lime carbonate by agitation. Warmth 

 and protection invited organic life, both plant and animal, which probably 

 occupied the lagoons in low forms that did not become fossil before 

 trilobites, the earliest that have been preserved, discovered the habitat. 



The description of the Man-t'o formation has thus far dealt with it 

 as it is developed in northern China. The red mud does not occur in the 

 south on the Yang-tzi'-kiang, where we saw the base of the Sinian, but the 

 strata which we suppose to be equivalent are thin-bedded gray limestones 

 which rest on a well-defined glacial till. The latter was seen only near the 

 village of Nan-t'ou, and we have named it the Nan-t'ou tillite. 



Nan-t'ou tillite. The Nan-t'ou glacial deposit occurs in longitude 

 in east, latitude 31 north, about 200 feet, 60 meters, above sea. It 

 evidently accumulated close to sea-level in early Sinian time, as it is 

 overlain by marine limestones of that age. At the base the plane of the 

 pre-Sinian unconformity is characteristically developed and covered by 

 a cross-bedded quartzite, which may have been either river deposit or 

 beach. The top of the quartzite is generally covered in the type locality 

 and a cultivated slope interrupts the section for 100 feet, 30 meters. Above 

 the terraced fields occur steep banks of tillite, a greenish rock, about as 

 hard as unweathered shale, of irregular hackly fracture, not stratified, and 

 containing pebbles and boulders of various kinds and sizes, many of which 

 are striated. The thickness seen is 120 feet, 36 meters. 



At the top of the tillite, beneath a cliff, is a well-exposed contact with 

 the overlying limestone. The tillite passes into a greenish shale, consisting 

 of the same materials, including characteristic pebbles, all rearranged by 

 water. This shale conglomerate is about 2 feet thick and grades into the 

 overlying limestone, the basal layer of a great thickness of Sinian. 



The facts clearly demonstrate the presence at this spot of a glacier 

 which gave way to marine waters and left a deposit of till that was slightly 

 washed by waves before it was buried beneath calcareous mud. 



The glacial deposit was seen only where it is exposed in the gorge of the 

 Yang-tzi'-kiang, beneath limestone cliffs. Both northward and southward 

 from the river the escarpment stretches beyond sight, and continuous 

 below it is the slope which, at the river, is formed of the tillite; but so 

 common a topographic feature as a slope below a cliff affords little ground 

 for inferring the extension of so unusual a deposit as an early Cambrian 

 till, and we limit ourselves to the positive statement of occurrence at 

 Nan-t'ou. 



