382 THE GEOLOGY OF MINNESOTA. 



[Geological structure. 



Juglans cinerea, L. Butternut. Rhus glabra, L. Smooth sumach. 



Rubus strigosus, Michx. Red raspberry. Rosa blanda, Ait. Rose. 



Symphoricarpus occidentals, B. Br. Wolfberry. Tilia Americana, L. Bass. 



Xanthoxylum Americanum, Mill. Prickly ash. Cornus. Different species. 



Salix. Different species. Ribes Cynosbati, L. Prickly gooseberry. 



Crataegus coccinea, L. Thorn. Celtis occidentalis, L. Ilackberry. 



Acer saccharinum, Wang. Sugar maple. 1'opulus monilifera, Ait. Cottonvvood. 



Acer dasycarpum. Ehr. Soft maple. CraUegus Crus-ealli, L. Cockspur thorn. 



Ulmus fulva, Mich. Slippery elm. Fraxinus sambucifolia, Lam. Black ash. 



Viburnum Opulus, L. High-bush cranberry. Primus Virginiana, L. Choke cherry. 

 Carya alba, Nutt. Shagbark hickory. 



The last is seen on land of M. B. Bullis, in Moscow township, near the county line. A. A. 

 HAKWOOD. 



Besides the foregoing, the following list embraces trees that are frequently seen in cultiva- 

 tion in Frteborn county. 



Juuiperus Virginiana, L. Red cedar. Pirus Americana, DC. Mountain ash. 



Populus balsamifera, L. var. candicans, Gray. Populus dilatata, Ait. Lombardy poplar. 

 Balm of Gilead. Robinia Pseudacacia, L. Locust. 



Larix Americana, Miclix. Hackmatack. Thuja occidentalis, L. Arbor vitse. 



THE GEOLOGICAL STRUCTURE. 



There is not a natural exposure of the underlying rock in Freeborn 

 county. Hence the details of its geological structure are wholly unknown. 

 It is only by an examination of outcrops in Mower county and in the adjoin- 

 ing counties of Iowa, together with a knowledge of the general geology of 

 that portion of the state, that anything can be known of the bed-rock of Free- 

 born county. In the absence of actual outcrops of rock within the county 

 there are still some evidences of the character of the rock that underlies 

 the county, in the nature and position of the drift materials. There is, be- 

 sides, a shaft that has struck the Cretaceous in the northwestern portion 

 of the county, in exploration for coal. 



Although the drift is heavy it lies in such positions that it shows some 

 changes in the surface of the bed-rock. It is a principle pretty well estab- 

 lished that any sudden great alteration in the rock from hardness to soft- 

 ness, as from a heavy limestone layer to a layer of erosible shales, or from 

 shales to more enduring sandstone, each stratum having a considerable 

 thickness, is expressed in the drift by changes from a rough and rolling, 

 more or less stony surface to a flat and nearly smooth surface, or v ice versa. 

 It sometimes happens that the non-outcropping line of superposition of 

 one important formation with another, either above or below, can be 

 traced across a wide tract of drift-covered country by following up a series 

 of gravel knolls or ridges that accompany it. or by some similar feature of 



