IN DETAIL 295 
347. The parts of a formation. All the parts which make up the struc- 
ture of a formation are directly referable to zonation and alternation, alone or 
together, or to the interaction of the two. The principles which underlie 
this have already been discussed under the phenomena concerned. It is 
necessary to point out further that the structure may be produced in several 
ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as 
primary and alternation as secondary, (4) by primary alternation and sec- 
ondary zonation, (5) by the interaction of the two, as in layered formations. 
Though all these methods occur, the first two are relatively rare, and the 
Fig. 76. Early (prior) aspect of the alpine meadow formation (Ca- 
rex-Campanula-coryphium), characterized by Rydbergia grandiflora, 
resulting structure comparatively imperfect. The typical structure of for- 
mations can best be made clear by the consideration of a prairie which be- 
longs to the fourth group, and a forest which represents the last. 
The major divisions of prairie and forest formations are regularly due to 
alternation. ‘There is an inherent tendency to the segregation of facies, 
arising out of physical or historical reasons, or from a combination of both. 
Not all formations show this, but it is characteristic of the great majority 
of them. The primary areas which thus arise have been called associations : 
they are naturally subordinate to the formation. To avoid the confusion 
which inevitably results from using the word association in two different 
