203 



characterizes most of the North of England dykes which remain to be 

 described. The pyroxene is almost colourless and frequently occurs in 

 good sized ophitic plates. The interstitial matter is very abundant. It is 

 frequently rendered almost opaque by brownish granular matter. It also 

 contains skeleton felspars, acicular microlites of felspar and grains, rods 

 or plates of iron-ore. 



The chilled margins of the dyke are formed of a compact rock 

 which effervesces freely with acid. Under the microscope this rock is seen 

 to be micro-porphyritic. The larger constituents are small columnar 

 felspars, frequently having bifid or ragged terminations, pseudomorphs 

 after a well-crystallized augite, and irregular grains of opaque iron ore. 

 The ground-mass is a confused aggregate of extremely minute felspar 

 microlites, specks of opacite and indistinct brownish granules. Iso tropic 

 glass may or may not be present. A considerable amount of calcite in 

 the form of crystalline powder is scattered through the rock. The 

 absence of well-formed augites in the central portions of the dyke coupled 

 with their presence in the marginal portions is a fact of some importance, 

 as apparently showing that the period of the consolidation of the augite 

 is dependent upon physical as well as upon chemical conditions. The 

 micro-structure of the marginal portions of the dyke is that of an 

 augite-andesite. 



The Tynemouth Dyke is exposed on the sea-coast at the base of 

 the rock on which the Priory stands. It runs nearly East and West. 

 The well-known Coley Hill Dyke, near Newcastle, is probably a portion 

 of the same igneous mass. The principal feature of the Tynemouth 

 and Coley Hill dykes is the occurrence of large porphyritic crystals, or 

 crystalline granular aggregates, of a felspar allied to anorthite (bytownite). 

 The analysis of these porphyritic elements is given on page 146, and a 

 description of their general characters on page 141. The groundmass 

 in which the porphyritic constituents are embedded is similar in all 

 essential respects to the general mass of the Hebburn Dyke. The 

 distribution of the porphyritic constituents throughout the mass of the 

 dyke is somewhat irregular. Some portions are almost entirely free 

 from them, and it is remarkable that these portions are usually 

 very rich in small spherical amygdaloids, now mainly occupied by 

 calcite. 



A few miles north of Tynemouth, near the villages of Seaton and 

 Hartley, several dykes are known. They run approximately parallel to 

 each other in a direction somewhat north of west and south of east. 

 Everywhere they present the same general characters. The unaltered 

 rock is dark, in some cases almost black in colour, and finely crystalline 

 in texture. Porphyritic elements are rare, but now and then crystals 

 of felspar, similar to those of the Tynemouth Dyke, may be recognized. 

 In some specimens the small spherical amygdaloids may be detected. 

 Alteration at the surface gives rise to brown colours, but in the coal- 

 pits these are never observed, the large amount of carbonaceous matter 

 causing the water to act as a reducing instead of an oxidizing agent. 



