384 LAMPLUGH : EARTH-MOVEMENT IN NORTH-EAST YORKSHIRE. 
bands, whether intercalated in a marine or in a fresh-water series, 
are perhaps the surest indications of ancient planes of horizontality, 
since they must have been accumulated at or close to the sea-level of 
the period and are frequently continuous over wide areas. Coral-reefs 
and similar zoogenetic beds composed of organisms that thrive within 
narrow limits of depth are likewise serviceable ; and we can find also 
other extensive marine deposits, particularly among those of the 
calcareous sandstone type, which possess faunal and lithological charac- 
ters indicative of an even sea floor. 
On the other hand, some limestones and most shales or clays, 
unless carrying an estuarine fauna, are not safe indications of level, 
as they may have been deposited at variable depths on a sloping or 
irregular floor. 
When the change of relative level within a given district has been 
considerable in the interval between the deposition of two originally 
horizontal beds, a definite unconformity may have been developed by 
the erosion of the uptilted portion of the lower band before the higher 
band was accumulated. More commonly, the displacement has not 
been sufficiently great to produce this result, and the inequality has 
been levelled up by the precipitation of thicker sediments over the 
depressed area ; so that the strata between the bands form a blunt 
wedge, of which the thick end indicates the area of greatest depression. 
When several index-beds of horizontality can be detected in an 
unbroken sequence of deposits, the intervening wedges of strata will 
show whether the movement was constant in direction, and will give 
its vertical measure between any two of the planes. 
Index-Planes in the Yorkshire Jurassic sequence. 
There are several planes within the Yorkshire Jurassic sequence 
for which, with certainty, we may postulate former horizontality, 
and others which we may place in the same category with fair con- 
fidence. They are fortunately so distributed as to give us an insight 
into events through almost the whole of the period ; and their evidence 
points consistently to the predominance of downward movement in 
one quarter from beginning to end, in spite of some minor local 
oscillations. This differential movement was superimposed upon the 
wider general movements affecting the area in common with other 
regions during the period. 
