28 LYELL'S ELEMENTS OF GEOLOGY. 



- Forms of Stratification. 



and rarely occur in a perfectly separate and pure form. Thus 

 it is an exception to the general rule to meet with a limestone as 

 pure as ordinary white chalk, or with clay as aluminous as that 

 used in Cornwall for porcelain, or with sand so entirely com- 

 posed of siliceous grains as the white sand of Alum Bay in the 

 Isle of Wight, or sandstone so pure as the grit of Fontainebleau, 

 used for pavement in France. More commonly \ve find sand 

 and clay, or clay and marl, intermixed in the same mass. When 

 the sand and clay are each in considerable quantity, the mixture 

 is called loam. If there is much calcareous matter in clay, it is 

 called marl; but this term has unfortunately been used so 

 vaguely, as often to be very ambiguous. It has been applied to 

 substances in which there is no lime ; as, to that red loarmisually 

 called red marl in certain parts of England. Agriculturists were 

 in the habit of calling any soil a marl, which, like true marl, 

 fell to pieces readily on exposure to the air. Hence arose the 

 confusion of using this name for soils which, consisting of loam, 

 were easily worked by the plough, though devoid of lime. 



Marl slate bears the same relation to marl which shale bears 

 to clay, being a calcareous shale. It is very abundant in some 

 countries, as in the Swiss Alps. Argillaceous or marly lime- 

 stone is also of common occurrence. 



There are few other kinds of rock which enter so largely into 

 the composition of sedimentary strata as to make it necessary to 

 dwell here on their characters. I may, however, mention two 

 others, magnesian limestone or dolomite, and gypsum. Mag- 

 nesian limestone is composed of carbonate of lime and carbo- 

 nate of magnesia : the proportion of the latter amounting in 

 some cases to nearly one half. It effervesces much more slowly 

 and feebly with acids than common limestone. In England this 

 rock is generally of a yellowish colour ; but it varies greatly in 

 mineralogical character, passing from an earthy state to a white 

 compact stone of great hardness. Dolomite, so common in many 

 parts of Germany and France, is also a variety of magnesian 

 limestone, usually of a granular texture. 



Gypsum. Gypsum is a rock composed of sulphuric acid, 

 lime, and water. It is usually a soft whitish-yellow rock, with 

 a texture resembling that of loaf-sugar, but sometimes it is 

 entirely composed of lenticular crystals. It is insoluble in acids, 

 and does not effervesce like chalk and dolomite, the lime being 

 already combined with sulphuric acid, for which it has a stronger 

 affinity than for any other. Anhydrous gypsum is a rare variety, 

 into which water does not enter as a component part. Gypseous 

 marl is a mixture of gypsum and marl. 



Forms of stratification. A series of strata sometimes con- 



