204 



The decomposed rock in the pits is in the condition of " white 

 trap." ( 



Under the microscope one recognizes long, narrow, lath-shaped 

 sections of plagioclase, irregular grains and plates of a colourless 

 pyroxene, and a certain amount of interstitial matter rendered more or 

 less opaque by crystals, rods and grains of iron-ore, brownish globulites, 

 and other bodies. Sometimes the interstitial matter is a brown isotropic 

 glass; at other times it is a colourless glass crowded with minute 

 brownish globulites ; in most cases, however, it is difficult to define its 

 precise character in consequence of its opacity even in very thin 

 sections. 



As a general rule the Hebburn, Tynemouth, Seaton, and Hartley 

 dykes are free from olivine, or contain it only in very small quantities. 

 We have now to notice a dyke, evidently belonging to the same series, 

 in which olivine appears to be tolerably abundant. It occurs near the 

 railway viaduct at Morpeth. The rock of this dyke is somewhat 

 blacker and more coarsely crystalline than that of the dykes hitherto 

 described ; it is also denser, its specific gravity lying between 2'88 and 

 2'89. Crystals and crystalline granular aggregates of felspar occur as 

 porphyritic elements ; but as they do not differ markedly in size from 

 the larger felspars of the groundmass, they are not so easily recognized 

 in the hand specimen as in the thin section. Olivine occurs both fresh 

 and in the form of serpentinous pseudomorphs. The other constituents 

 are similar in their character and relations to those of the Hebburn and 

 Tynemouth dykes. The augite is perhaps a little more abundant and 

 more prone to assume the ophitic form. 



Two dykes, presenting a type of structure somewhat different from 

 that of any of those previously described, are exposed on the east side 

 of the Tarret, near High Green, a farm-house about five miles N.N.W. 

 of Bellingham. The constituents of these dykes, as seen under the 

 microscope, are lath-shaped felspars, not very long in proportion to their 

 width, a pale brown pyroxene, occurring in grains, ophitic plates and 

 granular aggregates, large irregular plates of magnetic iron ore and apatite. 

 A very small amount of pale brown interstitial matter may be detected. 

 In the more northerly dyke two generations of felspar occur. The 

 felspars of the first generation are, however, few in number, and by no 

 means conspicuous. The specific gravity of the rock of these two dykes 

 lies between 2'9 and 2'93. The differences between the High Green dykes 

 and those of Tynemouth and Hebburn depend upon (1) the amount of 

 interstitial matter ; (2) the form of the felspars ; (3) the form of the 

 opaque iron ores; (4) the specific gravity, and (5) the chemical com- 

 position. In all these respects the H igh Green dykes approach the Whin 

 Sill which remains to be described. 



A very interesting feature illustrating the chemical processes involved 

 in the decomposition of basic igneous rocks may be seen at the northern 



(1) South Staffordshire Coal Field. JUKES, 2nd Edit,, p. 118. 



