l6 NEW YORK STATE MUSEUM 



the Paleozoic rocks of the Champlain valley. This has chiefly 

 expressed itself in the formation of a great number of normal 

 faults, both large and small. The Paleozoic rocks of the trough 

 are but little folded and usually but slightly tilted, but they are 

 cut into great slices by a series of normal faults which trend about 

 north-northeast, and which are themselves broken by occasional 

 cross faults. Most of these faults downthrow to the east, but at 

 the same time the upthrow side is given a tilt to the west, forming 

 a valley when combined with the upthrow side of the next fault 

 to the west. Occasional faults throw to the west, with the result 

 that a depressed structural valley called a graben is formed. 



The drop from the level of the Adirondack highland to the low 

 grounds of the Mohawk trough is produced by a series of these 

 great faults, all throwing to the east. 



Passing from the Mohawk rocks to those of the more easterly 

 troughs, one finds a quite different kind of structures. In the 

 first place the rocks are much more folded. The chief structures 

 here are also faults, but instead of being nearly vertical and normal, 

 they are quite flat and are overthrusts, along which great masses of 

 rock have been pushed westward for miles. Unlike the Mohawk 

 rocks, they now lie far from the area where they were originally 

 deposited and form a jumble of overthrust masses. Their structure 

 is exceedingly, often hopelessly, complicated. 



DESCRIPTIVE GEOLOGY 



BY H. P. GUSHING 



The exposed rocks of the two quadrangles are of Precambric, 

 Cambric and Ordovicic age, together with the unconsolidated 

 deposits of Quaternary age and the small exposure of extrusive 

 rock of unknown age north of Schuylerville. 



PRECAMBRIC ROCKS 

 The Precambric rocks of the Adirondack region, so far as 

 known, are comprised in an old series of sedimentary rocks, 

 named the Grenville series, which are the oldest known rocks of 

 the district, and in various masses of igneous rocks all of which 

 cut the Grenville rocks intrusively and are therefore younger. The 

 oldest of these igneous rocks is a granite called the Laurentian. 

 Later than this came a series of intrusions, anorthosite, syenite, 



