Mesozoic and Caenozoic Geology and Paloi ontology. 189 



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 superficial covering, 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 removed 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 by its re- 

 moval. 



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

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

 sea, marking, on its sides by a series of sea clitfs 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 above 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- 

 stratified and partl}^ assorted by water. In this vicinity, the regular 

 sequence is as follows : 1. Fine, uniformly grained sand, in some 

 places underlaid or replaced by stratified gravel. Marine shells in 

 the lower part. 2. Unctuous, calcareous clay, of gray, and occasionally 

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

 bowlder clay, filled with fragments of various rocks, usually, partially 

 rounded, and often scratched and polished. 



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

 or bowlder clay constitutes the greater part, but the sand often attains 



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



