PHYSICAL GEOGRAPHY. 15 



between 88° and 89°. Probably in nearly the whole tract 

 between the Rivers Wisconsin and St. Croix, and the Missi- 

 sippi and Lake Superior, it is very abundant. 



To the south of the lead region, that is, on Rock River, 

 on the east, and south of the Wabesepinicon, on the west of 

 the Missisippi, is a vast bed of bituminous coal (called by 

 Owen the great Illinois coal field), of a good quality, at no 

 great distance below the surface. The country is principally 

 of magnesian limestone formation. The rock is, for the most 

 part, covered with several successive layers of clay, each of 

 the depth of many feet, and is generally not found in digging 

 the wells of greatest depth. At the bluffs of the Missisip- 

 pi, however, and on some other streams, it outcrops. The 

 superstrata of clay are covered with a pure vegetable mould, 

 unmixed with other matters, of a depth from eight or ten 

 inches to three feet or more. In some localities, as at Iowa 

 City, are deposits of a fine madrepore or encrinitic marble. 

 The country has not yet been explored sufficiently to inform 

 us to what extent these abound, nor how great a variety of 

 minerals it contains. Mineral salt, and saltpetre, are to be 

 considered among them. 



The country has about all the varieties of forest trees 

 common to the same latitudes on this continent ; including 

 five or six species of oak, the walnuts, ashes, maples, elms, 

 hickories, locusts, mulberries, aspens, and poplars, one vari- 

 ety of which is very abundant, known as the cotton wood, 

 &c. There are very few birches, and the writer has not 

 seen any beeches or chestnuts. Of wild plums, the varieties 

 are almost endless ; many of them are good, some nearly 

 equal to the best cultivated plums, some indifferent. Iron- 

 wood is abundant on the bottoms. 



The prickly ash, hawthorn, grape, and gooseberry, are 

 among the shrubs and vines. The vegetation is not only 



