Mesozoic and Cceuozoic Geology and Palceontology. 287 



pliocene beds, would equal the period occupied by some deposits of the 

 Secondaiy age. Yet such a calculation would scarcely give the total 

 period accurately, since neither has the base of the lower blue clay 

 yet been reached, nor should the present alluvial surface be looked upon 

 as the last deposit of that epoch, or the prelude of the modern period; 

 since, the slopes of San Bernardino display a series of conglomerates and 

 gravels 200 feet above the level. of the nearest stream (Cajon creek). 

 These are coarse accumulations of primary pebbles and granitic clays, 

 which have been removed from every portion of the plain where it is 

 exposed. In the gorges and canons it still remains ; and wherever a 

 pass has been traveled, there it is found, as the superficialcovering, be- 

 tween 200 and 300 feet deep ; this, the last evidence of deposit of 

 the Post-pliocene period, has not been considered in calculation of du- 

 ration. Yet such a deposit must have existed over the plain, and must 

 have been I'emoved afterward; so that two additional periods would 

 still require to be added to make the calculation complete, namely, the 

 period occupied by the last deposit, and the period occupied b}' its re- 

 moval. 



Prof. J. W. Dawson* said that the mountain of Montreal, in 

 Canada, which rises 700 feet, forms a tide-gauge of the Post-pliocene 

 sea, marking, on its sides b}' a series of sea cliffs and elevated beaches, 

 the stages of gradual or intermittent elevation of the land as it rose 

 to its present level. The most strongly marked of these sea margins, 

 are at heights of 470, 440, 386, and 220 teet above Lake St. Peter, on 

 the St. Lawrence, or 450, 420, 366, and 200 feet abo\^e the river at 

 Montreal. 



The highest of these beaches contains sea shells of existing species. 

 Below the lowest, and at an elevation of about 100 feet above the river, 

 spreads the great Tertiary plain of Lower Canada, everywhere con- 

 taining marine shells, and presenting a series of deposits partly un- 

 stratilied and partly assorted by water. In this vicinit}^ the regular 

 sequence is as follows : 1. Fine, uniforml}^ grained sand, in some 

 places underlaid or replaced by stratified gravel. Marine shells in 

 the lower part. 2. LTnctuous, calcareous cla}', of gray, and occasionallj" 

 of brown and reddish tints. A few marine shells. 3. Compact, 

 bowlder cla}", filled with fragments of various rocks, usually, partiall}'' 

 rounded, and often scratched and polished. 



The thickness of these beds is at least 100 feet, of which the lower 

 or bowlder cla}' constitutes the greater part, but the sand often attains 



* Can. Nat. and Geo. vol. ii. 



