DYKE 



175 



through the superincumbent rocks often overflowing the newer strata; filling 

 fissures which may have been pre-existent, or may have been formed by the me- 

 chanical force of the fluid mass in its upward movement. The substances of which 

 dykes are composed is basalt, in one or the other of its varieties or trap in some of 

 its many forms. Dyke-matter is of different degrees of fineness, some of it being 



stono rocks of Kowley and other places. 



In the North of England the dykes have been very carefully examined. From the 

 ' Practical Treatise on Mine Engineering,' by G. C. Greenwell, we extract the following 

 notices of some of the principal dykes observed in the Newcastle coal-field : 



Many of these dykes passing through the coal, by dissipating the volatile bitumen, 

 bring it into an anthracitic state, and in some instances the coal is actually coked by 

 the heat of the intrusive mass. 



The main dyke running from near Cullecoates, in a south-westerly direction to 

 Hedley, is usually called the 90-fathom dyke, because it has a downthrow to the north 

 from 90 to 180 fathoms. 



The Mausoleum dyke (fig. 770) was discovered and passed through in the workings of 

 the low main seam at the depth of 53 fathoms in the Old Hartley Colliery, its position 

 being nearly underneath the mausoleum in the Seaton Delaval grounds ; hence Its 

 name. It is frequently called the ' Hartley Whin Dyke.' By it the strata are thrown 

 down to the north-east to the extent of 8^ fathoms. This dyke consists of two walls 

 of basalt of the thickness respectively, of 9 feet and 4 feet 4 inches, separated from each 

 other by a mass of the debris of other strata of the thickness of 8 feet 10 inches. 



In the neighbourhood of Dudley, in South Staffordshire, these overflows of trap, 

 upon the beds of coal, and in many cases through them, are strikingly exhibited. 

 Fig, 771 shows the passage of a dyke through the coal, the portions of the coal near 

 the trap being changed by its heat, or ' blacked,' as it is locally called, the coal losing 

 all its brilliancy. The late Mr. Jukes thus describes these intrusive dykes in the 

 coal of South Staffordshire : 



' All parts of these galleries, indeed, were riddled, as it were, with these trap veins 

 running here and there in the coal, and also in the sandstone, altering the coal to a 

 greater or less extent, according to the bulk of the trap, but -producing very little 

 apparent alteration in the sandstone. 



