Related Geological Conditions 35 



thicker strata above and below. No better example of such 

 can be adduced than the so-called "bone beds" of Silurian, 

 Devonian and succeeding epochs. These are often crowded 

 with entire teeth, scales or bones of well-recognized groups 

 of fishes, or with the crushed fragments of these. Extend- 

 ing often in stratigraphic continuity over hundreds of square 

 miles of territory; at times repeated in character, with a 

 few inches or even feet of strata intervening between them 

 till four or five bone beds may be recorded in a section 50 

 feet to 100 feet in thickness; they furnish evidence of the 

 teeming abundance of fishes to a degree that almost staggers 

 conception. Numerous cases of their occurrence will con- 

 cern us later, while it is with some of them that the 

 palaeontological record of fishes first opens. 



Remarkable features of practically all of these, "Bone- 

 beds" are that the rock-matrix is of an extremely fine and 

 often fissile texture; that it is often so hard and close- 

 grained as to give out a metallic ring when struck; and that 

 while in some cases the matrix is jammed with teeth scales 

 or bones, in others the matrix is finely laminated, and carries 

 between the laminae perfectly preserved and flattened fish 

 or other remains. 



Murchison, in describing the English Downton beds of 

 the Ludlow series says : "The highest member of the Ludlow 

 rocks is the most interesting Inasmuch as until recently It was 

 described by myself as being the oldest rock in which fossil 

 fishes had been found. The only exception Is that already 

 alluded to — the occurrence of a fragment of Pteraspis in 

 the central part of the same formation. The uppermost 

 Ludlow rocks also contain the earliest remains of land 

 plants. The lower layers of this zone, as seen at Ludlow, 

 are finely laminated, earthy, greenish-gray sandstones, con- 

 taining a few ichthyolites with several shelly remains 

 characteristic of the formation. It was the middle part 

 only of this band, or a gingerbread-colored layer of a thick- 

 ness of three or four inches, and dwindling away to a 

 quarter of an inch, which exhibited, when my attention was 

 first directed to It, a mottled mass of bony fragments, for 

 the most part of small size, and of very peculiar character. 

 These, with a few remains of shells and crustaceans, includ- 



