in Yorkshire and Durham. 35 
resulting from decomposing felspar, are often disseminated through 
these earthy masses, and enable us to separate them from other 
argillaceous materials with which they are sometimes in contact. 
It would be a laborious, and not a very profitable task, to attempt 
a minute account of phenomena like these, which vary with 
every different locality. 
It now remains to describe some of the effects produced by etets pro- 
the intrusion of the dyke. These effects will of course vary igor ne” 
with the substances which are acted on. In some of the quarries °“* 
«vhich have been already described, the trap passes through 
horizontal beds of slate-clay, and the changes produced by its 
presence are in all these cases strikingly similar. At Nunthorp 
and Langbargh* these beds of slate-clay belong to the great 
allum-shale formation (Lias), and are easily identified by the 
belemnites, pectinites and other characteristic fossils which are 
imbedded in them. On approaching the dyke they become much 
indurated, and are divided by a great many vertical fissures, 
which, when combined with the ordinary cleavage, separate the 
strata into rhomboidal fragments. In all such cases the rifts and 
fissures are coated over with oxide of iron. In other instances, 
the true horizontal cleavage entirely disappears; and the indu- 
rated masses might then be easily mistaken for beds which had 
been tilted out of their original position. The alteration pro- 
duced in the coal-shale at Gaundlass Mill is exactly analogous 
to what has been described, though not so strikingly exhibited. 
In the quarry at Barwick, on the right bank of the Tees, 
the vein of trap is well denuded, and the south side of the sec- 
tion exposes a great many horizontal beds of sandstone, which 
are separated into prismatic blocks by a number of natural 
transverse fissures. Close to the dyke this structure disappears ; 
* See Plate II. Fig. 2. 
E 2 
