36 NEW YORK STATE MUSEUM 



the Precambric surface between Glenfield and East Martinsburg is 

 due to the settHng of the mass on the south side of the fault. 



Extensive folding of the Paleozoic rocks nowhere occurs. Local 

 folds are sometimes developed but even these are rare. Such small 

 folds are best seen ^^ mile above the mouth of Mill creek (north 

 of Boonville) ; near Denley; and where the railroad crosses Sugar 

 river. At the latter place a syncline in the Trenton is perhaps the 

 best example of folding in the district. 



The folded structure of the Precambric rocks will be dealt with 

 under the heading " Gneissic structure." 



Ripple marks 



Small ripple marks are frequently present in the Paleozoic forma- 

 tions. Thus at several horizons within the Pamelia, Lowville and 

 Trenton limestones there are ripple marks which measure i or 2 

 inches from crest to crest and about ^ inch from trough to crest. 

 Special attention, however, is called to certain ripple marks of un- 

 usual interest in the Trenton. They are of unusual interest both 

 because of their large size and their occurrence in limestone. 

 These marks occur about 25 or 30 feet above the base of the 

 Trenton and are finely shown in the south bank of Sugar river a 

 short distance above the railroad bridge. The ripples measure 

 from 24 to 56 inches from crest to crest, and 4 to 7 inches from 

 trough to crest. They strike about n. 30° e. The ripple marked 

 layer varies in thickness from 2 to 9 inches and is a crystalline 

 and very fossiliferous limestone. Shale occurs immediately above 

 and below the marked stratum and the shale above thickens or 

 thins according to whether it rests upon the troughs or crests of 

 the ripples. The limestone layers both above and below rapidly 

 thicken and thin and are certainly of shallow water origin. 



Gneissic structure 



The precambric rocks, which are metamorphosed, igneous and 

 sedimentary masses, all exhibit the gneissic structure. This struc- 

 ture is best developed in the old Grenville sediments and least in 

 the syenite. In the areas of mixed gneisses it is also clearly shown. 

 The strike of the gneissic bands varies from north-south to almost 

 east-west, but the most common range is from n. 40° to 70° e. The 

 direction of dip of the foliation planes is either northward or 

 southward but prevailingly northward. The angle of dip is usually 

 high, varying from 50 to 80 degrees although just north of Lyon? 



