APPARENTLY CORRELATED TO THE CONDITIONS OE LIEE. 303 
periods during the progress of the gradual drying up of the lake. On each of these 
terraces Cockle shells are found in great quantities, having been left there when the 
water was at the level of the terrace. A series of specimens, therefore, taken from 
each terrace from above downwards, gives examples of the shells as they were at eacli 
stage during the progressive desiccation of the lake. On several of the terraces tlie 
shells are paired shells, with the ligaments more or less preserved, placed upon their 
oral surfaces, just as they were when alive, being kept in position by a crust of sand 
cemented with oxide of iron. Unfortunately, there is no reliable means of estimating 
the time which elapsed during the process of drying up. The intervals of time, how¬ 
ever, between the formation of the successive terraces were sufficiently long to enable 
the shells to acquire definite characters, especially of colour and texture, which made 
it easy to distinguish shells of any one terrace from those of the one above or below it. 
