proceedings of the polytechnic association. 319 



Hearing Hairs in Crustacea. 



M. Hensen states that, in the Decapod class of shell-fish, different sounds 

 cause different hairs connected with the auditory cavity to vibrate. 

 One hair will vibrate while all its neighbors remain quiet. The sense of 

 hearing- in these creatures is very acute. 



The Desert of Sahara. 



According to M. Martin, this desert embraces three distinct kinds — 1st, 

 the desert of the polateaux, consisting of large surfaces covered with gyp- 

 sum or plaster of paris; 2d, that of erosion; 3d, that of sand, resembling 

 waves of the sea rendered suddenly inmiovable. These waves are covered 

 in some places with a thin vegetation of shrubs and grass, which often 

 rapidly disappears under the drifting sands. The oasis is a plantation of 

 palms. The number of oases have greatly increased since the introduction 

 of Artesian wells. 



Geological. 



Dr. R. Stevens said the recent explorations in the Desert of Saliara by 

 M. Desor would lead us to infer that its geological age was about the same 

 as that of the sands of the Atlantic coast, beginning at the east end of 

 Long Island and extending to the capes of Florida. This has been ascer- 

 tained by recent discoveries in the desert of edible shell-fish, like the clam, 

 muscles, etc., such as are now found in the Mediterranean Sea, and this 

 settles the question among those acquainted with conchol5gy. The doctor 

 then directed attention to the primitive rocks, such as granite; some thirty 

 years ago it was supposed that these rocks were not truly primitive, but 

 that they had undergone a change. The cause of this cliange was a ques- 

 tion very much mooted. Before that time it was supposed that these rocks 

 had been thrown up by fire; afterward, that the change was of a purely 

 chemical nature. The query was as to the cause of the heat. Was it the 

 result of chemical action or from internal fires. A very prominent school 

 of geologists now aver that it was spontaneous, and that in fact the sedi- 

 mentary rocks were changed by action that took place within themselves. 

 Dr. Emmons, who made the geological survey of the north-eastern part of 

 the State of New York, expressed the opinion several years ago that the 

 primitive rocks of that period held fossils; and, in the Canadian survey, 

 similar indications ware soon discovered. Within the past year it has been 

 shown these were true fossils; these rocks which were termed "primitive" 

 are now regarded as sedimentary rocks, which have undergone a change 

 such as may be instanced in the quartz. He would boldly assert that there 

 is not a cabinet in the world which contains a specimen of quartz that has 

 been acted upon by fire. Such would show an entirely different density. 

 Tlie specific gravity of heated quartz is much lighter than any native quartz 

 we now have. The fossils alluded to are the same as those discovered in 

 the Adirondac mountains, and are known as infusoria, or, more commonly, 

 as rhizapods, a name given to that class which throw out from the shell 

 feet resembling rootlets; these are so small tliat they can hardly be seen 

 by the naked eye in the present seas. The speaker then illustrated upon 



