coked. Its lateral pressure has altered the cleavage of stratified 

 rocks from horizontal to the vertical for a few inches. 



Another curious point noted by Mr. Barrow is the way in which, 

 for a few yards, the beds dip into the dyke. It looks as though the 

 intrusive rock had contracted on cooling and dragged down the 

 adjacent rock with it. In no case is the dyke known to be in a 

 line of fault. 



In structure the dyke is an exceedingly compact hard crystalline 

 rock, grey or rather bluish grey in colour, weathering to a deep 

 brown. Being a most excellent material for macadamising, it is 

 extensively quarried for that purpose. 



The chemical composition as analysed by Mr. Stead, is as 

 follows : — 



This analysis and a lower specific gravity shows the rock to be 

 acidic in character rather than basic or basaltic. The absence also 

 of olivine tends to remove it further from a true basalt. Mr. J. J. 

 H. TeaU. describes it as an augite andesite ; for exhaustive infor- 

 mation upon the minute structure of the rock, I commend you to 

 that gentleman's work on British Petrography, 



We have the interesting fact that this dyke breaks through the 

 whin sill,becoming probably connected with the volcanic disturbance 

 which has left us the basalts of the north of Ireland and the 

 Hebrides. At the time we assume that this dyke was formed, the 

 west and north of Scotland was part of a chain of active volcanoes ; 

 the Isle of Mull is the fragment of one which, according to Professor 

 Jvidd, probably had a diameter at its base of nearly 30 miles, and a 

 height of from 10,000 to 12,000 feet. The highest point of what 

 remains of this mountain does not attain more than 3,000 feet. 



* From Teall on some of the North of England dykes. 



