GEOGNOSY AND GEOLOGY. 107 
by a mass of similar chemical character or composition. Where this is the 
case it is often difficult to decide whether the rock belongs to the isonomic 
or to the heteronomic, the passage from the one class to the other being 
effected by such forms. 
Series 1. Sandstones. 
Sandstones are conglutinates of fine grains, generally uniform in size. 
The part combined consists of quartz granules, which are either indefinitely 
angular or round. The cement is either a simple mineral or a mixture of 
various bodies. The principal kinds are : 
Quartz Sandstone, in which quartz grains are connected by a quartzose 
cement. The color is generally light ; grey, brownish, yellowish ; seldom pure 
white. Its hardness is considerable. 
Chalcedony Sandstone. This is of considerable hardness, as would 
naturally follow from its composition, consisting of quartz grains combined 
by chalcedony. Color grey, yellow, or blue. 
Argillaceous Sandstone. This is extensively distributed, and of great 
importance as a building material. The cement is argillaceous, and 
accordingly the rock, when breathed upon, emits the characteristic odor of 
this substance. Its color may be either light or dark, these being sometimes 
so combined as to produce markings. Its hardness is less than that of the 
preceding varieties. The clay is occasionally separate, in masses of a 
spheroidal shape, as in the well-known clay stones. Mica not rarely occurs 
as an ingredient, and then contributes to the lamination. 
Calcareous Sandstone. 'The cement here consists of carbonate of lime. 
Its colors are frequently similar to those of the preceding ; it may, however, 
always be distinguished by the effervescence produced by acids. The 
cement is rarely crystalline. 
Marl Sandstone. The cement is sometimes clay, sometimes lime marl. 
It therefore effervesces upon the application of acids, and emits an 
argillaceous odor when breathed upon. Colors white, green, grey, and red, 
these often darkened by carbonaceous particles. 
Iron-clay Sandstone. The cement is an iron-clay, frequently separated 
in clay stones. Its principal color is reddish-brown, in which white and 
grey not unfrequently produce markings. It is sometimes so thinly 
laminated that large plates may be obtained. 
Iron Sandstone. The cement is limonite or argillaceous oxyde of iron. 
The grain sometimes increases so much in size as to give rise to a true iron 
conglomerate. Colors generally dark-brown and yellow. 
Series 2. Conglomerates. 
The conglomerates are combinations of fragments of simple minerals or 
compound rocks, angular or rounded; the cement either a simple mineral 
or itself a conglomerate. 
Tron Conglomerate. Fragments of quartz, clay slate, and, at times, of 
other rocks, are combined by hydrated oxyde of iron. The cement is 
sometimes yellow, sometimes brown iron-stone, the pieces combined being 
at times so sparingly distributed, that the rock passes into limonite ; on the 
other hand, the fragments may be in such large proportion as completely to 
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