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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 impres- 

 sions 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 stratum of sand, it forms a deep 

 and narrow 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 sur- 

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

 According to Messrs. Whitney and Desor, these different 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 these different forms of im- 

 pressions, and their striking contrast with impressions of rain 

 drops from the same beach, mouth of Carp River, Lake Su- 

 perior. 



Mr. Teschemacher said, that he had seen fossil rain 

 drops, so called, with an elevated ridge crossing them ; an 

 appearance easily explained by Mr. Desor's hypothesis, but 

 incompatible with the supposition that they were caused 

 by rain. 



Prof. Agassiz said, that on the mud flats at Cambridge, 

 he had noticed impressions made in the way described by 

 Mr. Desor at Lake Superior. 



