GEOGNOSY AND GEOLOGY. 125 
11. Beds. 
A definitely limited rock mass, consisting of the same species of rock 
throughout, is called a bed, and mountain masses or formations are 
composed of a succession or superposition of such beds. These beds 
exhibit one primary difference, having reference to their relations of 
dimensions: they are either extended with tolerable uniformity in all 
directions, or else in one direction rather than another. In the first instance 
the components have a curved surface, or undefined angular form, while in 
the latter they approximate to the tabular. These beds are sometimes 
connected over extensive spaces, and sometimes they are interrupted. 
This interruption is either apparent or actual. The apparent is very 
interesting in a geological point of view; it especially occurs in trough 
formations. If we examine the map of the tertiary basin of Paris 
( pl. 44, fig. 6) and its section (fig. 7), we shall see that the chalk formation 
is interrupted at Bourges, Auxerre, Chalons, Rheims, and Laon, by the 
tertiary masses, disappearing at Paris, Melun, and Orleans, and coming out 
again at Chartres, Tours, Le Mans, Evreux, Rouen, and Amiens. This, 
however, is not actually the case ; the chalk is only covered by the tertiary, 
as seen by the section, where 1 indicates the tertiary, 2 the chalk, 3 the 
succeeding Jura formation. The bed 2 thus forms a trough or basin, in 
which the bed 1 has been deposited, hiding the other to a certain 
extent. 
The beds are either in immediate contact, or they are more or less 
separated from one another. In the latter case they are separated by the 
interposition of an inconsiderable mass. By their mutual contact they are 
brought into layers, which generally follow a definite order. Before 
considering this latter point it will be necessary to pay some attention to 
the mode of bedding. 
Beds of tolerably equal dimensions are often bounded by others only on 
one side; and, again, may be inclosed on all sides. The former then not 
seldom project from the latter. Most frequently the beds are laid, one on 
top or after another, forming various angles with the horizon. This 
condition of imposition or combination is known as the order of succession ; 
thus we say the bed A (pl. 43, fig. 4) succeeds B, B succeeds C, C succeeds 
D, &c. In fig. 3, also, a succeeds B. The study of this order is of great 
importance in the consideration of stratified rocks. 
When a plane of arrangement is more or less horizontal, and the beds lie 
one above the other, they are said to be imposed, or to cover one another ; 
if this be not the case, they are applied. When beds come one after the 
other, we have to examine whether the applied bed lies at an equal, a 
higher, or a lower level. 
When beds are imposed, it not unfrequently happens that they decrease 
as they ascend, thus giving rise to the formation of terraces like the Trapp 
Mountain on the Scandinavian peninsula. Overlapping exists when one 
oed overlies two or more others. Thus, if a bed of muschelkalk rest 
555 
