46 JOURNAI^ OP THE 



middle and southern Atkuitic, which has been carefully 

 studied by Mr. McGee. There is usually more or less 

 unconformity^ between the three or more formations of 

 Tertiary, and often they are not continuous over the 

 whole Tertiary field, but thin out and disappear from 

 some portions while they reach ^reat height and thick- 

 ness elsewhere. It is difficult to say more than there 

 were at least three mig-rations of the Tertiary coast- 

 line caused by uplift and sul)sidence which took place 

 rather unevenly but never causing* any considerable 

 transgression over the border line already described. 



It is reasonable to expect that the characteristic forms 

 caiised by the persistence of shore conditions at certain 

 levels would furnish evidence in the case of such recent 

 deposits as the Tertiary, and doubtless they would if 

 sufficient study had been made even of these terraces, 

 shore cliffs and raised beaches which are known to ex- 

 ist. Such shore marks would enter very prominently 

 into the investigation of the Quarter nary shore-line to 

 which our attention must now be directed. 



It should be remembered that there has been a suc- 

 cessive addition of essential land surface along the At- 

 lantic slope from Ca^X' Cod southward through all the 

 geological time from Triassicdown to Quaternary, and 

 a consequent recession of the j^hore-line eastward. 

 From Cape Cotl northward, liowever, the reverse has 

 to some measure been true, that is, there has been an 

 excess of subsidence over the constructive processes 

 l)y which all the deposits from the beginning of the 

 Mesozoic to the Quarternary, if formed at all, are now 

 buried beneath the sea. 



The most marked feature of Quarternary time was 

 the great ice invasion. A prodigious accumulation of 

 ice in the northern half of the continent was accom^mn- 



