202 



The Cleveland, Acklington, and Eskdale Dykes are thus seen to possess 

 the closest possible relations to each other. They are non-olivine bearino- 

 plagioclase-augite rocks, containing a certain amount of interstitial matter, 

 and possessing a lower specific gravity and higher silica-percentage (see 

 analyses p. 206) than normal dolerites or basalts. As already remarked they 

 belong in all probability to one of the later phases of Tertiary volcanic 

 activity. 



We have now to refer to some other dykes occurring in the North of 

 England, about the age of which it is necessary to speak with a considerable 

 amount of caution. 



Near the villages of Tudhoe and Hett, in Durham, a dyke of basic 

 igneous rock occurring in the Coal-measures has been worked for road 

 metal on an extensive scale. The course of the dyke is nearly E. and W., 

 sometimes showing a tendency towards the E.N.E. and W.S.W. In the 

 latter respect this dyke differs markedly from those of Cleveland and 

 Acklington, both of which tend towards W.N.W. and E.S.E. directions; 

 sometimes running N.W. and S.E. The rock is dark grey or bluish grey in 

 colour and either fine or medium grained. Its specific gravity varies from 

 2'94 to 2'96. It is composed of plagioclase, augite, magnetite, and a very 

 small amount of interstitial matter. The felspars belong to one generation. 

 They are strongly acted upon by hot hydrochloric acid in a few hours, and 

 belong therefore to a basic species. The pyroxene occurs in isolated 

 granules and also in granular aggregates. It is pale in colour and is some- 

 times seen to be penetrated by the felspar. The structure is, however, in 

 no case markedly ophitic. The iron ore in this rock appears to be 

 magnetite, as it gives rise to brown decomposition products and is removed 

 from the slide by hot hydrochloric acid. The interstitial matter occurs 

 only in very small quantity. It is brown, and gives a vague reaction with 

 crossed nicols. 



About two miles to the north of Tudhoe and Hett is another dyke of 

 similar composition, and between the two, at a depth of about 60 fathoms 

 occurs a horizontal sheet of similar material. This sheet has been proved 

 by mining operations to cover an area of 15 acres at least. It is associated 

 with the Coal-measures. The rocks of the two dykes and the intrusive 

 sheet have been described by Sir I. Lowthian Bell, and his analyses are 

 quoted below. 



The Hebburn Dyke occurs in the northern part of the county of 

 Durham. It crosses the Tyne at Walker and enters the county of North- 

 umberland. Its course has been traced by the numerous colliery workings 

 in the district. At Boldon Pit the rock of the main mass of the dyke is 

 very dark, almost black in colour, and uniformly crystalline in texture. 

 Here and there it contains small spherical amygdaloids, measuring on the 

 average about 1 mm. in diameter. These are occupied by calcite and 

 quartz. The specific gravity of the unaltered rock is 2 -84. Under the 

 microscope the rock is seen to consist of felspars, pyroxene, magnetite or 

 ilmenite, and a considerable amount of interstitial matter. The felspar 

 sections are usually very long in proportion to their width, a feature which 



