DEVELOPMENT OF THE APPALACHIAN GEOSYNCLINES 177 



lateral compression, when the geosyncline and the borderland were folded' 

 into the Appalachian polygenetic mountains. 



SAINT LAWRENCE GEOSYNCLINE 

 (See Maps, Figures 3 to 8) 



The northeastern half of the greater Appalachian geosyncline, extend- 

 ing: from eastern Kew York and Massachusetts to east of N'ewf onndland, . 

 is known as the Saint Lawrence sea. The strata along the northwestern 

 shore of the Saint Lawrence Eiver, and to the west of Lake Champlain 

 as well, still remain nndistnrbed. They are the materials farthest re- 

 moved from the source of main supply, the highlands of the New Brmis- 

 wick geanticline to the southeast and east. These deposits are chiefly 

 limestones of Cambrian, Ordovician, and Silurian ages and the thick- 

 nesses are usually small. Along the eastern side of the Adirondacks there 

 is less than 5,000 feet; about Quebec there appears to be less than 1,500' 

 feet; farther northeast, across the Mingan and Anticosti islands, there 

 is about 4,000 feet, and in southeastern Labrador less than 500 feet re- 

 mains. How far these shallow seas spread over the Canadian shield is 

 unknown, since this most positive part of IsTorth America has usually 

 been above the strandline and is now stripped of nearly all the marine 

 deposits which once rested on it. Ordovician strata are known, however, 

 about Lake Saint John and elsewhere in Quebec, and since those that 

 remain are of the kinds deposited far from shores, it appears safe to 

 postulate that most of the seas of the Saint Lawrence geosyncline spread 

 several hundred miles to the northwest of the Saint Lawrence Eiver. 



To the southeast of the great river and estuary, however, all the strata 

 of the Saint Lawrence geosyncline are in greatest confusion, being folded, 

 crumpled, and widely thrusted in superimposed sheets, and intruded by- 

 igneous masses nearest the geanticline. This superimposed structure is 

 due in smallest part to the orogeny of late Devonian time, when the 

 trough was, however, completely blotted out; the greatest deformation 

 came with the time of most marked compression and thrusting, during 

 the Pennsylvanian and Permian. All geologists have found it exceed- 

 ingly difficult to unravel the stratigraphic sequence here, and as well to 

 determine the thicknesses of the formations. The greatest thickness of 

 strata appears to be in the Gaspe area, Avhere there may have been as much 

 as 20,000 feet. In northwestern Newfoundland there appears to have 

 been 15,000 feet of subsidence, while to the southeast of Quebec and Mon- 

 treal the formations may attain a similar thickness. In northern Ver- 

 mont the total deposition may not exceed 8,000 feet, and in Massachusetts 



