RIPPLE MARKS EPRY. 317 



the harbor of this locality and stretching all along the island as far 

 as Ors, Le Chateau, and beyond. The layer of mud, constantly 

 accumulating by the accession of fragments of shell derived from 

 the oyster beds, attains a thiclmess of from 60 centimeters to 1 

 meter and more. In figure 19 is shown lying exposed to the sun at 

 low tide the surface of this desert of mud and the marks which indi- 

 cate the currents flowing from the Straits of Maumusson and 

 Antioche. It is impossible to recognize a trace of ripple marks there, 

 although the place is scarcely 200 meters from the point of Menson 

 and its ripple marks, and is affected by the same current. 



Ripple marks, then, do not occur in pure mud. They are only 

 formed on the sand, of whatever purity it may be. Although very 

 muddy, as especially in the same locality, on the beaches of An- 

 goulins, the sands cover it very easily. The bank with ripple marks, 

 which we have already noticed in Bretagne in the river Trieux, was 

 of a muddy sand, as is evidenced by the bright reflection of the sun- 

 light on its surface. 



It may be asserted that these undulations are nowhere so fine and 

 sharp as where the currents find in suspension in the water which 

 they propel enough clay to cement the always crumbling sandy ma- 

 terials of these ephemeral structures. If the sand is very pure, the 

 grains roll too easily on one another and are prevented from coming 

 to rest on the sides of the furrow. Such ridges consequently have a 

 wider base and proportionately less height than ripple marks of a 

 smaller size formed on bottoms that are slightly muddy. If the sand 

 is not only pure, but of very large grains, such as is encountered, for 

 example, at Penmark, at Guilvinnec, and at Langoz, or again south 

 of Concarneau from the point of Jument to that of Trevignon, then 

 the ripple marks are formed with great difficulty. They occur here 

 only under the influence of very strong currents and attain in this 

 case very remarkable dimensions. 



From this circumstance I am led to formulate the last of the 

 observations that I shall present, to wit, their dimensions are always 

 in direct relation to the force of the currents that form them. If in 

 the bay of Goulven, after having taken the view No. 1, with the 

 coast of Plouescat for the horizon line, I had made a half turn and 

 had advanced 200 or 300 paces in a new direction toward Brignogan, 

 I should have photographed ripple marks twice the size of those 

 shown in figure 1. It is in this place that the creek that serves as 

 the outlet of the marsh and bay passes through, much increased in 

 breadth and lacking depth, though still rapid. During the whole 

 time of the ebb, the entire strength of the current bears on this point, 

 and the ridges on the coarse sands transported there reach an excep- 

 tional size. 



