210 Mr. Tate on Basaltic Rocks. 



southern outcrop is in the Lime, a burn which flows into the 

 Tees. Above the Weel it is overlaid by a granular white 

 limestone ; and at Caldron Snout and High Force the river 

 cuts through it, and tumbles over cliffs, exposing a limestone 

 below it, which is metamorphosed, and white, and crystalline*. 

 The effects here are similar to what we have already seen in 

 Northumberland. 



BASALTIC VERTICAL DYKES. 



Besides the great lateral dykes of the Whin Sill, there are 

 several basaltic dykes in Northumberland which cut through 

 the strata nearly vertically, most of them running in a toler- 

 ably straight line, in a direction nearly from east to west. 

 The rock of which they are formed is similar to the Basaltic 

 Whin Sill, but generally finer in the grain and more altered 

 by contact with the adjacent strata ; the larger dykes usually 

 have dislocated and metamorphosed the strata, but the 

 smaller have had little effect. Of the following basaltic 

 dykes, we have some definite information; we begin with 

 the most northern : — 



The Cornhill Dyke cuts perpendicularly through beds of 

 the Tuedian formation, on the south bank of the Tweed, half 

 a mile below Coldstream bridge ; but it is not seen on the 

 opposite side of the river. It is traceable in a direction east 

 by north, a distance of seven miles, cutting also through 

 Mountain Limestone beds. The strata have been dislocated 

 and their relative levels altered by it, though the rocks them- 

 selves are very slightly changed. On the Tweed the dyke is 

 ten feet wide, but it widens eastward ; for at Melkington it 

 is eighteen feet, at Heaton Mill twenty-four feet, and at 

 Mattalees thirty-three feet wide. 



The Lindisfarne Dyke is one of the largest in the county, 

 and indeed has been erroneously described as part of the 

 Whin Sill, to which it has some resemblance, as it rises in 

 Lindisfarne or Holy Island, in high craggy hills of columnar 

 basalt. It crosses the south part of the island nearly from 

 west to east, and is seen two miles sea-ward, forming the 

 Plough and Goldstone rocks, on which the " Pegasus" was 

 wrecked. The castle crowns a high craggy basaltic hill, and 

 on the west side of the island the dyke is exposed in a high 

 cliff, and is there one hundred and twenty feet wide with a 

 slope 85° southward ; large blocks of limestone, highly 

 * Trans. Camb. Phil. Soc. vol. ii., p. 162. 



