460 Proceedings of the 



the specimen spread on paper, will show that there is no mis- 

 taking the die used for the " medal." On the same slab, at 

 the paper point, Dicholria viridis will be found pretty well 

 defined. After detaching this slab, and one or two smaller 

 pieces, my time was up, and I could examine no farther. The 

 stone herewith sent (such is the case with many of the printed 

 slabs) is coated by Ralfsia verrucosa, — a leather-like alga 

 common on our shores. This coating may be likened to the 

 chemical preparation in photography, the Ralfsia being the 

 sensitive part to be eaten away under the influence of the sun, 

 by its overlying, decomposing, and corroding brother. Thus 

 the impress is made ; and the stone, when washed by the next 

 flowing tide, will lose all the vegetable matter, both of the 

 decaying Desmarestia and the dissolved Ralfsia, — it being 

 carried away. The picture then shows beautifully on the 

 light ground of the yellow-coated slab. Not only does the 

 Desmarestia destroy the Ralfsia, it also dissolves some of the 

 rock ; and thus, as well as the depression left by the washed- 

 out alga, it is also engraved in the stone ; and probably this 

 depression is farther carried on from time to time by the car- 

 bonic acid in the sea-water ; and thus the more indelible it be- 

 comes. Even in rocks of a deep, dark gray colour, containing 

 little or no lime, and on which no growing alga is to be seen, 

 the Desmarestia imprints its form, by extracting the colour ; 

 and, although not so distinct as the prepared one, it is often 

 well defined. I saw some such on the coast. The lady of 

 the Rev. Mr Learmonth kindly showed me one, which a 

 quarryman had taken years before as a plant of the age of the 

 Old Red Sandstone. It retained all its markings quite fresh. 

 The rocks dip gently to the westward, and are exposed to 

 the full sweep and terrific -lash of the waves of the Atlantic. 

 The first, and I may say the only impress, must be quickly 

 done, to use a printer's term, "at one pull ;" for each returning 

 tide would certainly remove the weed, and leave not a trace 

 of the vegetable matter behind. When we take into con- 

 sideration the well-known property of destroying other algae 

 possessed by the Sporochnese, to which both the imprinted 

 plants belong, we cease to wonder at the eating away of the 

 Ralfsia. The extraction of the colour and dissolving the hard 



