296 The America?! Geologist. November, i899 
stratigraphic succession can not be deciphered. Important, 
then, as the stratified structure manifestly is, it would appear 
that some formal differentiation of its various phases could be 
advantageously made. 
It seems hardly necessary in this connection to note several 
other layered structures exhibited by rock masses that are not 
to be referred to stratification. These are acquired structures. 
They are shown as foliation in the slates and schistose rocks; 
as lamination produced by shearing along fault lines and thrust 
planes; and as flowage banding in lavas that have moved in 
cooling. 
Although the planes separating rock beds are so infre- 
(|uently considered any more than ordinary bedding phenom- 
ena, they are on that account none the less real. They not only 
have a wide theoretical interest, but they have an eminently 
practical bearing in geological classification. Each kind also 
has a distinct value in all problems of correlating rock ex- 
posures separated more or less widely geographically and in 
deciphering geological structures. 
For purposes of geological correlation and classification 
there are, among stratigraphic planes, at least four particularly 
useful distinctions that may be recognized. They correspond 
in a measure to the planes of division between the main 
groupings of strata that are considered in geological classifica- 
tion. These stratigraphic planes may be classed as (i) Ordi- 
nary bedding planes, (2) formational or terranal planes, (3) 
great planes of sedimentation and serial planes, and (4) erosion 
or unconformity planes. 
Bedding Planes, as here restricted in meaning, refer to the 
minuter layered characters of the sedimentary strata. They 
are the laminations in the shales, and the cross-bedding and 
ordinary bedded structures in sandstones and limestones. The 
layers that the term embraces, vary from the merest films to 
thick massive plates. Bedding planes being quite local in their 
nature are of little or no importance in the broader geological 
considerations. 
Terra)ial Planes may not always be distinguishable at first 
glance from bedding planes, and locally may coincide with 
them. In the field, when a number of exposures are compared, 
or an outcrop traced for any considerable distance, the planes 
