A. A. Michelson — Measurement Jy Light-inn;*. 115 



Geology, — The features of the surface geology to 

 which special attention was directed had reference to the 

 coarse ot the ice-flow and the distribution of bowlders and other 



drift. The conclusion is reached that the St. Lawrence river 

 extended, in pre-glacial times, from ii."> to 40 miles east of its 

 present valley, and that its channel, as well as that of many 

 tributary streams, Etchemin, Chaudiere, etc., was pre-glacial 

 and of considerable antiquity. Large areas of sand and clay 

 ur, and rock exposures are infrequent in the broad, flat 

 country east of the St. Lawrence. The pre-Cambrian ridges 

 abruptly rise from 1400 to 160<> feet, and then the surface 

 slopes gradually to the St. John river basin near to the Maine 

 boundary. 



Dr. Ells thinks that the theory of local glaciers instead of a 

 great ice cap appears to be supported by the direction of the 

 striae following the river valleys and the prevailing slopes from 

 the central highlands. This may be true, but if considered as 

 the closing phenomena of the glacial period after the great 

 ice sheet had retreated, the local glaciation is readily explained ; 

 and the theory of the cap still remains to explain the great 

 glaciation observed to the south, in New England, New York, 

 etc. 



The distribution of the bowlders of Laurentian rocks over 

 the surface is explained by submergence and floating ice carry- 

 ing the bowlders from the Laurentian hills south. Elevated 

 terraces are cited to show submergence. 



A systematic list of the fossils referred to in the report is 

 given by Mr. Henry M. Ami at the close of the report. 



Art. XVIII. — Measurement by Light-waves /* by Albert 

 A. Michelson. With Plate III. 



Almost every accurate measurement of any physical quan- 

 tity involves and usually depends upon a measurement of 

 length or of angle. In linear measurements the microscope is 

 usually employed, and in angular measurements the telescope. 



The use of these instruments depends entirely on the prop- 

 erties of the optical media in their relation to light-waves ; so 

 that in fact light-waves are now the most convenient means we 

 have for making accurate measurements; and it can readily 

 be shown that it is the very minuteness of the waves which 

 permits the extraordinary degree of accuracy already attained 

 in the use of optical instruments. 



* A lecture delivered at the Jefferson Physical Laboratory of Harvard 

 University. 



