On Fossil Rain Drops. 247 



sion surrounded by a rough crest, more or less elevated ac- 

 cording to the force of the rain. The fossil impressions on 

 sandstone, on tlie contrary, are generally flat and smooth. 

 Besides, there is hardly a shower in which the rain drops are 

 not numerous enough to cover the whole or nearly the whole 

 ground, whereas the fossil impressions are generally 

 scattered, and so few in number, that it seems almost im- 

 possible to ascribe them to rain. 



Mr Desor said, that whilst encamped on the border of 

 Lake Superior, they had several opportunities of studying 

 the action of the waves on the beach during a heavy surf, 

 when they are driven beyond their usual range. It was 

 noticed that when the waves retired from the higher part of 

 the beach, where the slope was less steep, there could be'seen 

 several kinds of impressions in the act of forming, some large 

 and flat, others small and deep (like those which on the sea 

 shore are generally ascribed to worms or shrimps), and 

 others likewise deep, but surrounded by a sort of annular, 

 smooth rim. These different kinds of impressions are all 

 produced by the same cause, operating in the same way, 

 namely, air-bubbles, which are formed in the waves of the 

 surf, when rolling over the beach. If an air-bubble 

 becomes buried in the sand, so that, in order to escape, 

 it has to make its way through the new formed stra- 

 tum of sand, it forms a deep and nan»ow hole. If the 

 air, instead of escaping at once, bubbles up several times, 

 then it raises around the hole a small and smooth rim, 

 which may be compared to a miniature crater of a volcano. 

 If, on the contrary, the air-bubble remains at the surface and 

 bursts, then it causes a flat and rather large impression. 

 According to Messrs Whitney and Desor, these difi^erent 

 forms of impression, arising from air-bubbles, are sufficient 

 to account for most impressions which have hitherto been 

 considered as the effect of rain. Such impressions of air- 

 bubbles are most perfect where the slope of the beach is 

 very gentle. Where the slope is more or less steep, the 

 sand becomes too much hardened, under the pressure of the 

 waves, to allow these delicate impressions to be produced. 



A sketch was exhibited, showing those diff*erent forms of 



