216 
THE RHAETIC BONE BEDS 
line shows equally remarkable variations (see diagram 
p. 215.) 
After a careful consideration of these different (and 
differing) sections, the only solution apparent is that the 
Bone Bed is not a regular or persistent bed, but consists 
of a number or series of thin sheets (or lenticules) of greater 
or less extent, coming in at various levels in the Lower 
Rhsetics, and having no connexion with each other, no 
instance having been noted of a lower bed joining a higher 
one, or vice versa. 
A further feature to be noticed is the remarkable simi- 
larity of their fossil contents. No matter where situated, 
whether a basal bed, or several feet above the base, the 
main group of fossils is identical, and this without the 
slightest reference to the character of the neighbouring 
beds, whether fossiliferous or not, thereby showing that all 
these Bone Beds, though separate from each other, have 
a common origin. 
Composition of the Bone Beds, 
We now pass to the composition of these beds. 
The true Bone Bed is always a hard rock ; that is, harder 
than the clays, sands, or loose marls in which it occurs, so 
that on exposure it tends to weather out. It is more or 
less crystalline, the usual crystals being calcite, selenite, 
and iron pyrites. The only exception to the crystalline 
character is when it occurs in a sandy bed, for then it 
is usually an indurated sandstone. (This statement applies 
to fresh sections, as exposed or weathered beds frequently 
break up.) 
It is mainly composed of organic remains of Fishes, etc., 
and it frequently (in the basal beds) contains fragments 
of the underlying rocks. The matrix is mainly calcareous 
