[BLis] CANADIAN GEOLOGICAL NOME.N-CLAÏURE 29 



Medina to the base of the Potsdam sandstone presented an unbroken 

 sequence, both on grounds of stratigraphy and from the contained fossils, 

 so that the divisions laid down in 1881 could not be maintained in 

 the field. As a consequence, the Calciferous and Potsdam sandstone, 

 the latter being now regarded as the basal portion of the former, have 

 been transferred to the lower portion of the Cambro-Silurian system, 

 which has also received the name Ordovician, adopted from the British 

 Survey, while the Levis and a part of the Sillery, which were held to 

 represent the Calciferous and Potsdam were also similarly or in part 

 removed from their supposed position in the Cambrian. The terra 

 Potsdam, for the portion of the Cambrian below the Potsdam sand- 

 stone, was superseded by the use, in part, of the term St. John group 

 or Primordial, and also by the employment of the term Georgia series 

 for the lowest beds of that system as developed in Quebec, east of the 

 St. Lawrence Elver, in which latter is probably to be included a por- 

 tion of the loM^er Sillery formation ; while the reddish slates of that 

 formation, formerly known as the Lauzon, may presumably be consid- 

 ered as the downward extension of the Levis beds, and in consequence 

 may be regarded as the equivalent of the Potsdam sandstone. 



The middle divisions of the Palœozoie in this scheme retained the 

 same names as given in the original nomenclature of Logan as stated in 

 the Geology of Canada, 1863; but the lowest divisions of the scale, com- 

 prising the rocks of the Huronian and Laurentian systems were material- 

 ly changed. Each of these was divided into three portions, a lower, 

 middle and upper. In this arrangement of the Laurentian, it will be 

 seen therefore, that Dr. Selwyn receded somewhat from the position he 

 assumed in his official paper of 1877-78, when he proposed to limit the 

 Laurentian to the lowest series of granite-gneiss, though no reason is 

 given in the accompanying text for this change of opinion. 



Within the last fifteen years, the study of these oldest rocks of the 

 continent has been undertaken with renewed vigour and with a fixed 

 determination to arrive at a true understanding of the structure and re- 

 lations of the several rock formations, which have, under various names, 

 hitherto been included under the comprehensive term Laurentian. The 

 work in this direction has been greatly facilitated by the recent opening 

 up of many areas by means of railways, which were previously accessible 

 with great difficulty; so that the structure of many portions, where im- 

 portant problems exist, can now be readily studied in detail. Li ad- 

 dition, the microscope has been largely employed in determining the 

 composition and origin of many of the larger rock masses which are in- 

 cluded in this great series of crystalline rocks. These investigations in 

 the laboratory have been supplemented by a careful investigation in the 



