18S7-88.] All Ancient Lake-deposit in Queen's Park. 135 



One peculiar feature is, that dichroic crystals under this liglit 

 have their prevailing colour intensified — as in the red of 

 platino-cyanide of magnesium ; and iu every position of the 

 prisms and selenites, crystals of this class adhere firmly to the 

 one colour, to the entire exclusion of all other polar tints. 

 The reason of this is obvious, when you consider that, in the 

 case of a crystal vmder polar dark-ground, the light, before 

 reflection, must take the colour of the medium it passes 

 through, thus becoming monochromatic. This fact is pointed 

 out by Tyndall ; and the example we here find is particu- 

 larly interesting and instructive to students of polarised light. 

 In closing, permit me to indicate that, by arranging the 

 microscope for polar dark-ground illumination, it will be 

 found tliat a very large number of slides that may have 

 become comparatively uninteresting under ordinary manipula- 

 tion will be found possessed of new interest ; and by 

 attention to the management of the instrument, and care- 

 ful observations of the many optical effects at command, it 

 is possible to acquire a more intimate acquaintance with the 

 various subjects than by mere book study or any other means. 



[Several coloured diagrams of diatoms, enlarged from plioto-micrograplis, 

 were exhibited in illustration of the above paper.] 



\.—AN ANCIENT LAKE-DEPOSIT IN QUEEN'S 

 PARK. 



By Messrs J. A. JOHNSTON and J. LINDSAY. 

 {Read Feb. 22, 1888.) 



FoK some months past extensive excavations have been in 

 progress in the Queen's Park, in connection with the making 

 of a new main-drain, and many interesting sections have thus 

 been exposed, all helping to reveal more clearly the successive 

 geological changes of which this well-known locality has been 

 the scene. At one portion of the workings, near Holyrood 

 Palace, the attention was particularly arrested by the masses 

 of shell-marl which the workmen were throwing out of the 



