250 ASSINNIBOINE AND SASKATCHEWAN" EXPEDITION. 



to show a perfect sectional view of its components, with- 

 out the risk of changing in the least degree their relative 

 positions. The clay was cut away until a perpendicular 

 wall was left, varying from ten to twenty feet in height, 

 according to the locality. Wedges were then inserted at 

 the top of the artificial cliff, about two feet from its edge, 

 and driven into the clay until a mass, frequently two feet 

 broad, fifteen or twenty feet long, and twelve or eighteen 

 feet deep, separated and fell. The fresh surface thus ex- 

 posed was necessarily quite natural in every respect, not 

 having been touched by the tool of the workman or 

 changed by exposure to the weather, and during the years 

 1855 and 1856, a large area of sectional surface was laid 

 open to view. Two varieties of blue clay exist in the 

 neighbourhood of Toronto, forming deposits quite distinct 

 from one another. The deposit in question overlies rocks 

 of Silurian age, which are exposed in many localities 

 on the lake shore and on the banks of the rivers near 

 the city ; it rests upon a blue argillaceous shale, easily 

 recognized as constituting in fragments of different 

 sizes, a large proporton of the substance of the blue 

 clay. 



The thickness of this deposit of blue clay varies from 

 ten to twenty-five feet ; its upper surface is irregular and 

 undulating, and upon it reposes, in some places, stratified 

 sand and yellow clay, in others, unstratified yellow clay. 

 Eesting on the sand or yellow clay, another kind of blue 

 clay occurs, differing, however, essentially from the blue 

 clay which lies at the base of the whole. The lower or 

 inferior blue clay contains quartz sand and small rolled 

 pebbles of granitic rocks, a considerable proportion of 

 blue shale containing fossils belonging to the Silurian 

 shales upon which it rests, and frequently large fragments 



