298 TJie Amenca?i Geologist. November, i.sflu 
In the area towards the sea the two series may be clearly separ- 
able, while among the shore deposits no such division plane 
might be suggested. Moreover, it might there remain long 
unnoticed were not the great plane traced directly from the 
limestone district. 
Again, an erosion plane may often coincide with a great 
plane of sedimentation, representing as it does a hiatus in local 
deposition. Whenever it occurs an episode or chapter in the 
local or regional physical history is missing. The erosion 
plane often stands for a time of base-leveling, when heavy de- 
posits for the formation of limestones are laid down very near 
shore. When the land sinks again, the calcareous succession 
remains unbroken. Yet, a new cycle of sedimentation is in- 
augurated. The unconformity plane projected seaward 
through the limestone succession is still a horizon in the latter' 
that is one of the most natural ones in the classification of pro- 
vincial deposits. 
Erosion Planes, as already intimated, have a special signifi- 
cance. They are more or less uneven, and indicate a strati- 
graphic break. At such a horizon, an entirely new cycle of de- 
position begins, the faunal succession is broken, and the rock 
sequence above bears no relation to that below. The extent of 
the erosion plane varies widely in dififerent cases. It may be 
very local, covering only a few hundred square miles, or it may 
stretch out over the greater part of a continent, crossing from 
one geological province to another without interruption. 
The erosion plane is, in stratigraphy, called an luiconform- 
ity ; and as a phenomenon of unusual significance by itself, may 
be further considered. Its importance as a means of primary 
geological classification is given at length elsewhere. 
While in the unconformable relations of two sets of rock 
layers a stratigraphic hiatus is generally recognized, the phe- 
nomenon is usually looked upon more as a mere structural fea- 
ture, than as representing a momentous event in the phvsical 
history of the region. The term as most rightly applied refers 
to a stratigraphic break caused by an erosion interval. So far 
as the structural relations of the strata are concerned, two very 
distinct kinds are recognized. In the one the stratification or 
bedding planes are essentially ])arallel and in the other they 
make a considerable angle with each other. These are, how- 
ever, only special cases of the general problem. 
