484 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV, 
more and more notched and irregular, until at last the beach seems 
to be dotted over with little, flat, dome-shaped mounds, or as if the 
ridges of the ripple-mark had been furrowed across. These modifica- 
tions may be due to the partial effacement of the ridges by subse- 
quent action of the water agitated by wind blowing from a different 
quarter. Such indications of shallow-water conditions may often be 
observed among old arenaceous deposits, as in the Cambrian and 
Silurian rocks. In like manner we may frequently detect, among 
these formations, small isolated or connected linear ridges (ril]- 
marks) directed from some common quarter, like the current-marks 
frequently to be found behind projecting fragments of shell, stones, 
or bits of sea-weed on a beach from which the tide has just retired, 



Fic. 196.—PLAN AND SECTION OF Fic, 197.—Srctions or RIrpLE-MARKS. 
RIPepLEeD SceRFACE. . 
On an ordinary beach each tide usually effaces the ripple-marks 
made by its predecessor and leaves a new series to be obliterated by 
the next tide. But where the markings are formed in water which is 
always receiving fresh accumulations of sediment, a rippled surface — 
may be gently overspread by the descent of a layer of sediment 
upon it and may thus be preserved. Another series of ripples may 
then be made in the overlying layers, which in turn may be buried 
and preserved under a renewed deposit of sand. In this way a 
considerable thickness of such ripple-marked strata may be 
accumulated, as has frequently taken place among geological 
formations of all ages. 
Sun-cracks, Rain-prints, Vestiges of former Shores— 
One of the most fascinating parts of the work of a field-geologist 
consists in tracing the shores of former seas and Jakes, and in 
endeavouring thereby to reconstruct the geography of successive 
geological periods. There are not a few pieces of evidence, which, 
though in themselves individually of apparently small moment, 
combine to supply him with reliable data. Among these he lays 
special emphasis upon the proofs that during their deposition strata 
have at intervals been laid bare to sun and air. 
The natureand validity of the arguments founded on this evidence 
will be best realized by the student if he can make observations at 
the margin of the sea, or of any inland sheet of water, which from 
time to time leaves tracts of mud or tine sand exposed to sun and 
