GEOLOGY OF OGDENSBURG 55 



syenite, granite and gabbro, have wide extent in the Adirondacks, 

 particularly on the east and south. They aided in altering and 

 metamorphosing, as well as in breaking up and destroying, the older 

 Grenville and Laurentian. The small masses, mapped as syenite, 

 on the Ogdensburg quadrangle, we refer to this second period 

 of eruptive action. 



The latest of the Precambrian rock series of the upper lake 

 region is accompanied by great flows and sheets of trap. The trap 

 dikes of late Precambrian age in the Adirondacks are of similar 

 rock and are naturally correlated with them. Only one such dike has 

 been found in the Ogdensburg region, but they are abundant farther 

 up the river, in the islands. Not only do they cut all the other 

 Precambrian rocks, but they are entirely unmetamorphosed, indica- 

 tive of a long time gap between them and the older rocks. 



There is no evidence that any of these later series of Precambrian 

 sediments wxre ever deposited in the Adirondack region, though it 

 is entirely possible that some of them may have been laid down, and 

 subsequently completely removed by erosion. There is no evidence 

 to controvert the statement that the Adirondack region was a land 

 area throughout all the great lapse of Precambrian time following 

 Grenville deposition, and that this portion of its history is one of 

 erosion rather than of deposition. A great thickness of Grenville 

 and of igneous rock was worn away, and the region was reduced 

 to the condition of a comparative plain, whose surface irregularities 

 were of a minor sort, and seem not to have exceeded 200 feet in 

 amount. Within this minor degree, however, the surface was fairly 

 rough, the weak rocks worn down into valleys and the more resist- 

 ant ones projecting as low hills and ridges. The intrusion of the 

 traps came toward the latter end of this long period. 



CAMBRIAN TIME 



The erosion period just mentioned involved the greater part of 

 Cambrian time in addition to the long Precambrian interval. But 

 in the latter part of the Cambrian all four sides of the Adirondack 

 region became depressed and deposits began to form on the old 

 erosion surface. Deposit began on the northeast, with coarse con- 

 glomerates, followed by sand, forming the initial deposits of the 

 Potsdam sandstone. These early deposits have furnished no marine 

 fossils and strongly suggest continental formations. In their lack 

 of thorough decay they also suggest climatic aridity. As time went 

 on the deposits gradually extended to the south and the west of 



