358 W. Upham — Diversity of the Glacial Drift. 



responding to the diaphragm in the eye-piece, while with the 

 objectives alone they are of course much larger. Objectives 

 are used ranging from 3 inches to 4 mm focus ; with the latter it 

 is barely possible to make micro felsitic structure visible. The 

 rings and brushes of crystals in converging polarized light and 

 other facts of optical mineralogy are well shown. For the 

 projection of ordinary lantern-slides the microscope is replaced 

 by a projecting lens and slide holder and at this short distance 

 the light is so powerful that the projections are visible by ordi- 

 nary daylight, a great convenience in a continuous lecture. 

 The arc-lamp is also used for this purpose in a large lecture- 

 room with a great gain in amount of light and convenience 

 over the oxyhydrogen light. 



The lamp is also used for microphotography and is occa- 

 sionally useful in other ways. The dynamo current, properly 

 regulated can be used in place of the usual battery for excit- 

 ing the electro magnet in the magnetic separation of the iron- 

 bearing minerals of rocks, with the advantage of easy variation 

 in the strength of the pull exerted. 



Harvard University, Petrographical Laboratory, Feb., 1894. 



Aet. XXXIX. — Diversity of the Glacial Drift along its 

 Boundary ; by YVarren Upham.* 



Recency and probable Brevity of the Glacial period. — The 

 recession of the ice-sheet at the end of the Glacial period in 

 the northern United States and Canada and in Great Britain 

 seems to have been separated from the present day by a Post- 

 glacial or Recent epoch of only about 6,000 to 10,000 years, as 

 made known by the observations and reasoning of N.'_ H. 

 Winchell, Gilbert, Andrews, Wright, Mackintosh, Prestwich, 

 and others. This conclusion, and the uniqueness of the Ice 

 age, standing quite alone as a strange episode of geologic his- 

 tory, unexampled besides in all the very long Cenozoic and 

 Mesozoic eras, forbid our longer reliance upon the once gene- 

 rally accepted astronomic theory of Croll, Geikie, and Ball, 

 that the accumulation of the ice sheets was due to terrestrial 

 conditions springing from the earth's relations to the sun dur- 

 ing a period of increased eccentricity of the earth's orbit from 

 about 240,000 years to 80,000 years ago. Dr. Croll's theory 

 supposed glacial epochs to recur alternately in the northern 

 and southern hemispheres each 21,000 years during the astro- 



* A paper presented before the Geological Society of America at the Boston 

 meeting, Dec. 29, 1893. 



