LIMITATIONS OF PRE-CAMBRIAN NOMENCLATURE 329 
where they have been deposited they are very commonly swept 
away in part by later erosion, before succeeding series are laid down. 
Moreover, the pre-Cambrian surface rocks are to a large extent 
volcanic flows or land sediments, and on this account are much more 
discontinuous than sediments of marine origin. 
Mode of origin of formations.—This criterion is of limited appli- 
cation; for sediments originating in the same way may be deposited 
during different geological epochs, and likewise sediments originat- 
ing in different ways may be deposited contemporaneously in 
adjoining localities. It might be of value in the correlation of 
certain uncommon types, such as glacial deposits, which generally 
occur only at long intervals in geological time. 
Relationship to batholithic intrusions.—The relationship of the 
pre-Cambrian surface rocks to the great epochs of batholithic 
invasion is of great assistance in correlation and may possibly 
eventually prove to be the most important of all the criteria used 
in the classification of the pre-Cambrian rock series into major 
groups; for geological investigation throughout the world has shown 
that batholithic intrusions are an accompaniment of mountain- 
building movements in the earth’s crust, and are thus directly 
related to the great regional changes in rock structure, to regional 
metamorphism, and to the uplifts which give rise to the great 
erosion intervals which form the dividing lines between the great 
pre-Cambrian terranes. Some of the applications and limitations of 
batholithic invasion in rock correlation are included in the following: 
Batholithic massifs are co-extensive with the mountains they 
underlie, and since mountains are generally extensive and linear 
the massif should also be extensive and linear. The extent of the 
outcrop of the massif will depend, of course, on the extent to which 
unroofing has been carried. In the Rocky Mountains, for example, 
unroofing has apparently only begun; in the Coast Range batholith 
of the Pacific Coast, on the other hand, unroofing is almost com- 
plete; and in some of the pre-Cambrian batholiths of the Canadian 
shield not only is the unroofing largely completed, but the batholith 
has also been reduced to base level. 
If two batholithic massifs have been intruded in a given region 
the younger may displace the first. Hence the conspicuous 
