136 



CLEVELAND BASALT DYKE. 



€d by the drying or shrinking in of the rocks. This dyke in its 

 course intersects very different formations, viz. the transition or me- 

 talliferous limestone, the coal district, and the upper secondary strata 

 of lias and oolite. The different organic remains in these formations, 

 as well as their position, prove that they were consolidated at distant 

 periods of time. Indeed, the geologists who maintain that dykes 

 "were formed as before described, are ready to admit the distant eras 

 of these formations. The transition or metalliferous limestone, and 

 the lower strata must have been completely consolidated, long before 

 the upper secondary strata were deposited ; and the causes which 

 might dispose the upper strata to shrink in, cannot be supposed to 

 act on the lower rocks. It is also to be remarked, that in the lower 

 rocks, situated to the west, the breadth of this dyke is more than 

 twenty yards ; but at Sillow Cross, where I measured it, it is not 

 more than ten yards : this dyke must, therefore, become wider as it 

 descends. It must also have been filled with basalt at the time of its 

 formation, otherwise it would have contained numerous fragments of 

 the rocks which it intersects. 



The effects of this basaldc dyke on the different rocks through 

 which it passes are truly deserving notice. When it comes in con- 

 tact with limestone, the limestone is often found granular and crys- 

 talline, a fact the geological importance of which will be subsequently 

 adverted to. Where it crosses the coal strata, and comes in contact 

 with the seams of coal, the substance of the coal is for several feet 

 converted into soot. At a greater distance from the basalt, the coal 

 is reduced to a coke or cinder, which burns without smoke, and with 

 a clear and durable heat. At the distance of fifty feet from the 

 dyke, the coal is found in its natural unaltered state. It is particu- 

 larly remarkable that the roof immediately over the coal is lined with 

 bright ciystals of sulphur. In some situations in the same county, 

 the shale, in contiguity with basaltic dykes, is converted into flinty 

 slate or jasper, and the sandstone is changed to a brick colour. There 

 is another great basaltic dyke in the same district, which crosses the 

 western extremity of Durham from Allenheads to Burtreeford on the 

 river Tees, hence called the Burtreeford Dyke. It throws down the 

 strata on the west side of it, one hundred and sixty yards. 



Dykes, being generally impervious to water, they obstruct its pas- 

 sage along the porous strata, and occasion it to rise ; hence it fre- 

 quently happens that numerous springs make their appearance along 

 the course of a dyke, by which it may be detected, when there is 

 no other indication of it visible on the surface. 



Basakic dykes intersect both primary and secondary rocks, but 

 they every where present indications of their action on the adjacent 

 rocks. At Nigg, near Aberdeen, I examined a basaltic dyke on the 

 coast, which intersects a rock composed of gneiss ; the dyke is 

 about thirty feet in width. Where the basalt is in contact with the 

 gneiss, it becomes nearly compact, and approaches to the character 



