THE PERMIAN AND TRIASSIC SYSTEMS. 711 



Magnesian limestone. This characteristic rock is well developed in a continuous band, 

 extending from the Tyne southwards through Yorkshire, and descending into the counties 

 of Derby and Nottingham. Towards the northern and southern extremities the strata 

 rise into a series of round-topped hills ; but the centre sinks to the general level of the 

 country. The appearance of stratification is very distinct, and several varieties of structure 

 occur some beds being granular, others imperfectly crystalline, and others cellular. In 

 the latter variety, abundant in the county of Durham, where it bears the name of the 

 honeycomb limestone, the cells are lined with crystallised carbonate of lime. The rock, 

 as its name imports, is a compound of the carbonate of lime and the carbonate of 

 magnesia, in various proportions ; but calcareous beds are common, in which there is 

 little or no admixture of magnesia with the lime. It forms one of our most durable 

 building stones, and has been largely used in the construction of some of our finest 

 edifices, as York Minster and Westminster Hall, which have admirably withstood the wear 

 and tear of the atmosphere. It is now employed in the erection of the new houses of 

 Parliament, derived from the Bolsover quarries in Derbyshire. An analysis of the stone of 

 York Minster gives 



Carbonic acid - - 47 '00 



Lime - - - - 33-24 



Magnesia - - - - 19-36 



Iron and clay - - -40 100 



The stone of Westminster Hall contains about 2 per cent, less of magnesia, but the 

 proportion is often far greater, amounting to more than 50 per cent. Magnesia in minute 

 quantities is very extensively distributed ; it occurs more largely in the mountain lime 

 stone, oolites, and chalk ; but is only found in such abundance in the limestone of this 

 system as to become a characteristic. The two constituents of the rock, the carbonates of 

 lime and magnesia, some suppose to have been deposited contemporaneously ; but Von 

 Buch considers the magnesian limestone to be a metamorphic change of common limestone, 

 effected by the disengagement of vapours containing magnesia from plutonic masses. 

 But it has been justly remarked, that, according to this theory, we ought to expect all 

 limestones to become magnesian in the neighbourhood of igneous rocks, which is far from 

 being the case. 



Gypsum. This sulphate of lime, one species of which is commonly known under the 

 name of alabaster, frequently appears in association with the red marls and sandstones 

 of the upper part of the series. It occurs in detached nodules of fibrous structure, in 

 horizontal seams equally fibrous, varying from one to three inches in thickness, and in 

 amorphous granular masses capable of being worked into columns. The red marly 

 banks of the Trent exhibit beautifully white and translucent seams of gypsum ; and at 

 Chellaston, not far from the borders of the river, it appears in a granular mass, from 

 which the beautiful pillars at Kedleston Hall, near Derby, were wrought. Its origin has 

 been referred by some to segregation from the surrounding sedimentary matter ; but its 

 common connection with rock-salt, wherever the latter mineral is found, is considered 

 by others as indicating its formation from the same cause. 



Rock-salt. The mineral, chloride of sodium, is not peculiar to the series of deposits 

 under notice, for salt springs occur in the coal-measures, and in strata belonging to the 

 modern volcanic period ; and salt is procured from the chalk and oolite systems. Former 

 writers, therefore, were too hasty in referring the production of all the saliferous deposits 

 upon the face of the globe to the geological era of the new red sandstone. Still, saline 

 strata clear, white cubically crystallised masses of salt, and brine springs issuing from 

 such rocks are so remarkable and frequent in it, as to be justly regarded as one of its 

 distinguishing features, and hence the common application of the term saliferous to the 

 entire series. By far the finest example of rock-salt in Europe, on account of its position 

 at the surface, is at Cardona, a small town in the interior of Catalonia, sixteen leagues 



