THE LOWER SILURIAN ROCKS. 549 



AVliilst in minor details the Lower Magncsian varies much as to its 

 lithologicai characters even individual horizons not showing any 

 great constancy in this regard all parts of it have some features in 

 common, which serve to distinguish it from the newer limestone 

 formations of the region. In general, it may be said that the Lower 

 Magnesian is a rnagnesian limestone, varying in composition from 

 varieties that contain not more than one or two per cent, of insoluble 

 ingredients, to those that are for the most part sand. An analysis 

 has been given, a few pages back, of one of the purest phases, whilst 

 in the detailed descriptions of this chapter, numerous determina- 

 tions of the quantity and nature of the insoluble ingredients of 

 both pure and impure varieties will be found. These in the 

 purer kinds are exceedingly fine and clay-like, whilst in the less 

 pure they are usually quartz sand of varying degrees of fineness, 

 the constituent grains always much rolled. The analysis above 

 cited, as also others made for the survey, do not show the car- 

 bonates of calcium and magnesium in the right proportion to make 

 a true dolomite, the first-named carbonate being always in excess. 

 Older analyses show a nearer approach to the composition of dolomite. 

 The purest kinds have usually a grayish-white color, a minutely crys- 

 talline texture, and marked conchoidal fracture. More commonly the 

 white back-ground is blotched with yellow, whilst other layers again 

 occur, in which the buff color is uniform. These are usually charac- 

 terized by a close, even, granular texture, which may be due to an 

 admixture of sand grains, or may characterize a quite pure lime- 

 stone. 



The bedding of the Lower Magnesian varies much. Usually those 

 layers from 40 to 60 feet above the base are the heaviest, the indi- 

 vidual layers running sometimes to a thickness of 4 to 6 feet, whilst 

 the bedding lines are exceedingly indistinct. In the upper and lower 

 portions of the formation, the layers are usually much thinner, and 

 more distinct, although commonly quite irregular, very rough- sur- 

 faced and internally porous, with drusy cavities that are lined with 

 calcite and dolomite crystals. Occasionally, however, the lower layers 

 are exceedingly regular, being obtainable in large, smooth-surfaced, 

 compact, and finely granular slabs. Highly concretionary layers, some 

 of which appear even to have a brecciated structure, occur at many 

 different horizons in the formation, the structure sometimes affecting 

 in the highest degree a layer not more than a foot thick, whilst above 

 and below, for many feet, no trace of a concretionary structure is ap- 

 parent. In other cases much greater and less well-defined thicknesses 

 are affected in this manner. One of the most prominent features of 



