128 _ GEOGNOSY AND GEOLOGY. 
formed within a certain definite period of time. These characters have 
reference to structure, to condition of aggregation, and to the included 
fossils or organic remains. Such a combination of beds, exhibiting these 
features, is called a formation. 
In such a formation we distinguish with reference to the importance 
of the included beds; principal measures, which always exist; secondary, 
which seem to accompany the last, and generally are of no great extent ; 
and subordinate, which do not always occur in the formation, but are 
limited to restricted spaces here and there. Single series of deposits within . 
a formation, agreeing in more special characters, give rise to another 
distinction into groups; these, not unfrequently, again being composed of 
individual rock species, which, in turn, may be built up of strata. On the 
other hand, entire formations may stand in a certain relation to each other, 
giving rise to their collocation into systems. 
Although a formation or a group generally possesses the same petrogra- 
phical character in all parts of the earth, yet there may be exceptions in 
certain cases. Certain groups of strata may be entirely different from 
others, and yet be of contemporaneous origin. When such an abnormal 
condition occurs we have to deal with a representative or a geognostical 
equivalent. 
14. Subordinate Strata. 
A rock measure is rarely so constituted as to consist entirely of the same 
geognostical species; measures also frequently occur which must be 
considered as subordinate, whether standing in a definite structural relation 
with the primary measures or not. In the former case they lie between the 
beds of rock connecting them with each other. | 
The ordinary subordinate beds, or those included between the 
approximately parallel surfaces, exhibit, in general, the same relations as 
were found to exist in the case of stratification. Thus certain coals and 
many iron ores occur in such beds. These are generally of nearly equal 
dimensions, and approximate in a greater or less degree to the spherical or 
spheroidal form. Forms of indefinite surface also occur, and this not very 
rarely. f 
The ore beds of the north, as the copper and iron pyrites’ beds at Fahlun, 
the ironstone at Arendal, &c., occur in masses of the above-mentioned 
character. 
Here belong also those matters which fill fissures and clefts in rocks. 
This matter may be of very different character in different cases; it may 
have been ‘introduced by a washing in from above, or by the injection of 
abnormal matter from below (pl. 53, fig. 14). These two varieties are 
readily distinguishable. The subordinate beds already considered, all stand 
in a certain connexion with the rock mass; there are others, however, in 
which this is not at all the case. These are of various shapes, ellipsoidal, 
spherical, &c., and vary in size from an inch to many feet. They are 
ither partly or entirely filled up. The masses which fill the spaces 
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