GEOLOGY AND PALEONTOLOGY OF THE SCHOHARIE VALLEY 93 



material will be deposited near the shore and the finer at a greater 

 distance from the shore in direct proportion to the fineness of the 

 grain. Thus the impalpable powder or rock flour maj^ be carried 

 out to a great distance. In general we have close to the shore the 

 rubble deposits, which may be angular or more or less waterworn 

 to the condition of completeh^ rounded and smooth pebbles. In 

 size this rubble deposit may vary from small pebbles up to boulders 

 many feet in diameter. From the consolidation of such material we 

 obtain "rubble-rocks." At a somewhat greater distance from shore 

 the sands are deposited, which may also var^^ in size from less than 

 a pebble to that of a barely recognizable fragment, and may be 

 either angular or rounded. Consolidation of this material pro- 

 duces sandstones. Finall}- the impalpable rock flour is deposited 

 only in quiet water, and by its consolidation produces mudrocks. 

 From Avhat has been said regarding the loci of deposition of these 

 various rocks it will be evident that the mudrocks alone are likely 

 to show uniformity in bedding, thickness etc. The sandrocks and 

 conglomerates, while as a whole remaining of uniform thickness 

 will show consdderable diversity in their individual beds, cross- 

 bedding, ripple marks, and lenslike thinning and thickening of 

 beds being common characteristics. 



The chemical composition of these types of fragmental or clastic 

 rocks is of course determined by that of the parent ledges from 

 which the material is derived. It will be pure silica if all but 

 the quartz is removed, which, if the texture is coarse, produces 

 silicious conglomerates, a type of rock best represented by the 

 quartz pebble conglomerates, though other quartz rubble rocks 

 also occur. Pure quartz sandstones and pure quartz mudrocks 

 are also produced when the texture is arenaceous and muddy 

 respectively. Most of the silicious rocks of the Schoharie region 

 are rendered more or less impure by the admixture of clay or iron, 

 but a nearly pure quartz. sandstone is found in the basal Siluric 

 sandstone. When the parent rock is limestone or an organic 

 deposit of corals or shells, such as coral reefs growing at a dis 

 tance from the shore, the clastic rock of that locality may consist 



