454 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS 



• 



newer at; the south, [n latitudes 85° to 87° a double interruption to 

 this arrangement occurs. The line of Devonian outcrop swings 250 



miles (100 km.) southward in :i great loop about the Cincinnati upward 

 arch; the Silurian outcrop swings an equal distance northward about 

 the Michigan downward arch; and between the two arches there is a 

 belt in which the dips are northward. The most resistant members of 

 the Paleozoic are the Trenton limestone, at the base of the Silurian, and 

 the Niagara limestone, near the top of the same system. Between 

 these are Shales and Soft sands of the dtica, Hudson River, and Medina 

 series, and above the Niagara are equally soft shales of the Salina 

 Hamilton, and Chemung series. The basins of lakes Michigan and 

 Huron arc carved from the monocline of soft rocks above the Niagara, 

 where it sweeps around the .Michigan syncline, and are thus made to 

 embrace the coal basin. Lake Erie lies in a trough carved from the 

 same monocline where its trend is nearly normal. Lake Ontario lies 

 in the monocline below the Niagara limestone, where its course is nor- 

 mal, and the same monocline, where it curves about the Michigan coal 

 basin, holds Georgian and Manitoulin bays, dependencies of Lake 

 Huron, and Green bay, a dependency of Lake Michigan. It is believed 

 that the excavation of the basins was accomplished partly by rains 

 and rivers during pre- Pleistocene and interglacial times when the 

 district stood at a higher level than now, and partly by Pleistocene ice 

 currents. As to the relative importance of the two agencies geologists 

 are not agreed, and it may safely be said that the determination of the 

 question belongs to the future."'- •"' , 



The surface features in Michigan depend largely on the drift, con- 

 sisting of ground moraine traversed by numerous marginal moraines. 

 Lakelets, ponds, and swamps are numerous. On both sides of the St. 

 Clair river, connecting Lake Huron with Lake St. Clair, the till is over- 

 lain by laminated Champlain clays, here called Brie clay, and the 

 surface of the country is smooth. 



From Sarnia to Hamilton, in the Province of Ontario, Dominion of 

 Canada/ 4 the route continues on the eastern limb of the Michigan syn- 

 cline, gradually descending in the geologic scale through the Devonian 

 and Upper Silurian. The Hamilton shales are succeeded by the Corn- 

 iferous limestone near London; the Onondaga follows, and at St. George 

 the Guelph, an upper member of the Niagara limestone. The main 

 mass of the Niagara is met at Capetown, the underlying Clinton at 

 Dundas, and the Medina at Hamilton. Nearly the whole country is 

 heavily sheeted by drift deposits. The Erie clay, which occupies the 

 entire surface for the first hundred miles and appears at intervals 

 beyond, is a laminated calcareous clay, with erratic pebbles and boul- 

 ders, but no fossils, apparently the deposit of a great lake at the 

 margin of the ice. Smaller bodies are traversed of the Algoma sand 



