169 



with this opinion. Dr. Jackson said he did not ascribe a\iy 

 mysterious power to the primary formations in banishing 

 cholera, but he thought this disease more likely to prevail 

 over the more recent and tertiary rocks, from the character 

 of the water drank by the inhabitants in such localities. 



Dr. C. T. Jackson read a paper on the Mirage of Lake 

 Superior of the months of July and August, 1847. 



During these two seasons this phenomenon was often witnessed 

 by Dr. Jackson. His theory of the mirage was as follows : The 

 water of Lake Superior is very cold, the temperature seldom 

 rising above 50° Fahrenheit, even in the hottest weather. In the 

 summer season the temperature of the air in the forests near the 

 shore is frequently as high as 90°, or even higher. Dr. Jackson 

 supposes that a current of warm air loaded with moisture is 

 blown from the shore, and coming in contact with a stratum of 

 cooler air, in contact with the Lake surface, a fih-n of moisture 

 is condensed above it, which acts as a mirror to reflect the 

 objects beneath. 



Dr. J. B. S. Jackson gave an account of a recent visit he 

 had made to Mr. Marsh's collection of Ornithichnites. 



In connection with this subject Dr. C. T. Jackson said, that on 

 the shores of Lake Superior he had seen some remarkable bird- 

 tracks made under his observation by the American Raven. 

 The peculiarity of these tracks consisted in the elongation for a 

 number of inches of the impression of the middle toe. This was 

 produced by the bird's trailing this toe on the sand as it slowly 

 walked over the surface. Some of the Ornithichnites present this 

 singular disproportion in the length of the track of the middle 

 toe, which can hardly be accounted for in any other way. This 

 mark of the toe is curved inwards also, as it is in the Ornith- 

 ichnites, a natural result of the gait of the bird. 



Dr. Gould gave descriptions with drawings of several 

 new species of shells brought home from the United States 

 Exploring Expedition, as follows: — 



CoLUMBELLA VALGA. T. ovato-lanccolata, sub-distorta, fulvo- 

 marmorata et lineolis crebris rufis cincta, ad apicem rosacea; 



