internal Structure of the Magnesian Limestone. 67 



in the last variety, a red or purple tinge predominates : but these colours pass 

 through every variety of shade ; in some places are only seen in the form of 

 cloudy spots, and in others disappear altogether. The rock then becomes a 

 brown or grey micaceous sandstone, which sometimes cannot be distinguished 

 from the gritstone beds of the coal-measures. 



4thly. Fine-grained sandstone, much less coherent than the preceding, and 

 less regularly bedded. In composition and colour it sometimes resembles the 

 most ordinary varieties of new red sandstone. 



5th. Nearly incoherent sand. In this state it is very extensively developed, 

 and sometimes alternates with the preceding variety. These incoherent 

 masses are seldom much tinged with red oxyd of iron, but more frequently 

 exhibit a grey or yellowish brown colour. They sometimes contain small 

 spherical calcareous concretions ; and the upper portion occasionally becomes 

 calcareous and cellular, and passes insensibly into the superior deposit of 

 limestone. 



6th. Sandy micaceous shale, often variegated with stains of a red or purple 

 colour. This variety not unusually alternates with some of the preceding. 

 The argillaceous earth which produces this variety is hardly ever in such 

 abundance as to entirely destroy the ordinary type of formation : for there is 

 perhaps not a single locality where it could be mistaken for a characteristic 

 mass of slate-clay subordinate to the coal-measures. Near the upper part of 

 the formation it is often associated with red marl and soft red micaceous slaty 

 sandstone. 



7th. Marls, much varied both in colour and composition. They have gene- 

 rally a red tinge, or are variegated with red and purple blotches. They are 

 frequently interposed between the beds, but they are chiefly developed at the 

 higher part of the formation, immediately under the yellow limestone. In that 

 situation they are of very common occurrence, though seldom of any great 

 thickness. Notwithstanding their extent, and the many sections in which they 

 are exposed, I never observed in them any beds of fibrous gypsum like those 

 which characterize the upper marls of the new red sandstone ; but they con- 

 tain, in a few rare instances, crystals and crystalline nodules of selenite*. 



It appears from the preceding details that iron, either in the form of a hy- 

 drate or red oxyd, is commonly diffused through all the subordinate parts of 



are extensively quarried for troughs, coping-stones, and coarse flagstones, &c. &c. At Hart Hill, 

 near the southern extremity of the same county, some of them are used as a rough building stone, 

 and the finer beds are ground down into scythe. stones. 



* The most remarkable instance of this kind which fell under my own observation occurs ou 

 the right bank of the Nid, a little above the bridge which leads from Knaresborough to Harrow- 

 gate. 



k2 



