26 THE MODERN PERIOD. 



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 tlie succeeding tide, which deposits upon them a new layer 

 of mud. Hence we find on splitting open a slab an inch or more thick, 

 on the upper surface of which the marks of recent rain occur, that an 

 inferior layer, deposited perhaps ten or fourteen tides previously, 

 exhibits on its under surface perfect casts of rain-prints which stand 

 out in relief, the moulds of the same being seen in the layer below." 



After mentioning that a continued shower of rain obliterates the 

 more regular impressions, and produces merely a blistered or uneven 

 surface, and describing minutely the characteristics of true rain-marks 

 in their most perfect state, Sir Charles adds : — 



" On some of the specimens the winding tubular tracks of worais 

 are seen, which have been bored just beneath the surface. Some- 

 times the worms have dived beneath the surface, and then reappeared. 

 Occasionally the same mud is traversed by the foot-prints of birds 

 {Tringa minuta)^ and of musk rats, minks, dogs, sheep, and cats. 

 The leaves also of the elm, maple, and oak trees have been scattered 

 by the winds over the soft mud, and having been buried under the 

 deposits of succeeding tides, are found on dividing the layers. When 

 the leaves themselves are removed, very faithful impressions, not only 

 of their outline, but of their minutest veins, are left imprinted on 

 the clay." 



We have here a perfect instance, in a modern deposit, of phenomena 

 which we shall have to notice in some of the most ancient rocks 5 

 and it is only by such minute studies of existing nature that we 

 can hope to interpret those older appearances. In some very ancient 

 rocks we have impressions of rain-marks, or their casts, on the under 

 surface of the overlying beds, quite similar to those which occur 

 in the alluvial mud of the Bay of Fundy. In these old rocks, 

 also, and especially in the coal formation, w^e find surfaces netted 

 with sun-cracks precisely like those on the dried surfaces of the 

 modern mud flats, and faithful casts of these taken by the beds 

 next deposited. A still more curious appearance is presented by 

 the rill-marks produced by the flowing of the recedhig tide, or of 

 rain, down inclined surfaces of mud. The little streamlets flowing 

 together into larger channels, form singular patterns, which may be 

 compared to graceful foliage or to the ramifications of roots, and which 

 have often been mistaken for fossils. In the following figures (Figs. 

 1, 2, 3) I have endeavoured to represent the surface of a small 



