134 



BASALTIC DYKES. 



The thickness of dykes varies from a few inches to twenty or 

 thirty feet or yards ; in some instances they exceed three hundred 

 feet. The extent to which they stretch across a country has seldom 

 been explored beyond the mining districts, where a knowledge of 

 them is important, on account of the disturbances which they occasion 

 in the strata. 



The intersection of coal strata by dykes is represented Plate IV, 

 fig. 2. and 3. C. C. and D. D. Dykes generally decline a little from 

 a vertical position ; and, as before stated, the depth to which they 

 descend is unknown. 



The strata are almost always thrown down on one side of a dyke^ 

 and elevated on the other ; but the dislocation is not proportioned to 

 its breadth. There is a fault extending from Whitly, in Northum- 

 berland, to Greenside and Sandgate in Durham, which has thrown 

 down the strata on the north side one hundred and eighty yards; this 

 is a comparatively narrow fissure filled with clay. A great basaltic 

 dyke in the same county, which is seventeen yards wide, has only 

 produced a dislocation of twelve yards. 



The whole series of strata which have been raised above the sur- 

 face on one side of a fault, have sometimes entirely disappeared, 

 and the ground on each side of it is on the same level. See Plate 

 IV. fig. 2, 3. 



Trap dykes, and basalt dykes are generally harder than the rocks 

 that they intersect ; and when the latter are partly decomposed, often 

 remain, forming vast walls of stone, that rise above the surface of 

 the ground. There are walls of this kind in the counties of North- 

 umberland and Durham, running along the country several miles. 

 Dykes also extend into the sea, and form reefs of rocks ; and when 

 they cross the beds of rivers they form fords, and sometimes hold 

 up the water and occasion cascades, of which there are numerous 

 instances on the river Tees. In the interior of North America, ba- 

 saltic walls of great extent were discovered by Messrs. Lewis and 

 Clark ; the walls were composed of columns of basalt arranged hori- 

 zontally, and were at first supposed to be artificial constructions. 

 Where basaltic dykes are of considerable thickness, the hardness of 

 the stone varies in different parts ; sometimes the inner parts are 

 harder, and sometimes softer than the outer, the substance in the 

 dyke being divided by seams or partings. This may be distinctly 

 seen at Coaly Hill near Newcastle-upon-Tyne, where a large basalt 

 or whin dyke cuts through the coal strata, and rises to the surface. 

 The stone, being hard, is quarried for the roads along a line of sev- 

 eral hundred yards, forming a deep trench, sufficiently wide to admit 

 a cart road through the quarry, between the sides of the dyke. 



The basalt of the dyke is intersected by fissures, and divided into 

 variously shaped masses. In one part of the dyke, it appears to 

 graduate into an indurated ferruginous clay, which is in some places 

 divided into minute, well defined pentagonal prisms. The dyke had 



