﻿Fucoids of the Cincinnati Group. 



127 



MODERN MUD MARKINGS. 



As preliminary to a consideration of these fossil marks, it will be neces- 

 sary to study some of the markings found on the mud banks of rivers, or 

 on ocean beaches when the tide has ebbed. The tracks or marks produced 

 by animal forms will naturally be different now from what they were during 

 the Silurian age. But those produced by natural causes simply, as, for in- 

 stance, the action of rain or the water of rivers on mud, or by the daily 

 ebb and flow of the tide, may be considered as at least similar, both then 

 and now. 



Some of these marks have been observed on sea beaches by Sir Charles 

 Lyell. In his "Prin. of Geol/' (Vol I., p. 327), he remarks, in reference 

 to impressions of rain-drops on the mud flats of the Bay of Fundy, as 

 follows: "When a shower of rain falls, the highest portion of the mud- 

 covered flat is usually too hard to receive any impressions; while that 

 recently uncovered by the tide near the water's edge is too soft. Between 

 these areas a zone occurs almost as smooth and even as a looking-glass, on 

 which every drop forms a cavity of circular or oval form; and if the shower 

 be transient, these pits retain their shape permanently, being dried by the 

 sun, and being then too firm to be effaced by the action of the succeeding 

 tide, which deposits on them a new layer of mud. Hence we find on split- 

 ting open a slab an inch or more thick, on the upper surface of which 

 marks of rain occur, that an inferior layer, deposited perhaps ten or twelve 

 tides previously, exhibits on its under surface perfect casts of rain prints 

 which stand out in relief, the molds of the same being seen in the layer 

 below." 



Precisely the same thing can be seen on the muddy banks of rivers after 

 a flood, and when a shower of rain falls before the mud is entirely dry. 

 My own observations show that the impressions are large, circular and 

 distinct. (Plate V., figure 1.) If the shower was only a light one, the 

 impressions remain distinct and separated. But if heavy, they are more 

 or less irregular; and in the case of very heavy rains they become entirely 

 obliterated, and the surface is marked with many ri<lls along which the 

 water has run; and on a gentle slope a larger channel will carry oft' the 

 water to the main stream, or into some depression. On quite soft mud. 

 close to the edge of the stream, the impressions are never so distinct. The 

 same impressions and casts referred to by Lyell, I have seen on splitting 

 open slabs of mud which had dried on the shore. 



