193 



this respect they differ from the basalts. The augite occurs as definite 

 crystals or as irregular masses. The felspars frequently penetrate the 

 augite, thus showing that, as a rule, the latter constituent was the last to 

 form. Granules of augite are rare in this group, whereas they are 

 extremely common in the basalts. The dolerites occur as intrusive masses 

 which frequently exhibit very interesting phenomena at their junction 

 with other rocks. . The change as the margin is approached is thus 

 described by Dr. GEIKIE : " The irregular plates of augite gradually give 

 place to minute granules. The same change is observed in the plates of 

 iron ore, in which case the minute specks group themselves as club-shaped 

 bodies. Felspars retain their form but become smaller ; they often tend to 

 arrange themselves with their longest diameters parallel to the bounding 

 surfaces, but remain quite distinct right up to the margin. Sometimes the 

 oxides take the form of a geometrically perfect network of interlacing rods. 

 Where the invaded rock is a sandstone the junction is sharp and the sandstone 

 is apt to break away; where it is an argillaceous shale a thorough welding of 

 the two rocks takes place. In such cases the dolerite at the actual junction 

 shows a still further diminution in its component particles. It becomes a dark 

 opaque rock, which in thin slices under the microscope is found to be a 

 mottled or curdled segregation of exceedingly minute black grains and hairs 

 in a clear, glassy matrix, in which the augite and felspar are not individual- 

 ized. But even in this tachylite-like rock perfectly formed and sharply 

 denned crystals may be observed." Good illustrations of the change which 

 takes place at the margins of intrusive masses of dolerite may be observed 

 at Hound Point, in Linlithgowshire, and at the Salisbury Crags, near 

 Edinburgh. 



The term basalt is limited by Dr. GEIKIE to the contemporaneous lavas 

 of basic composition. They consist of a compact or finely granular- 

 ground-mass through which crystals of plagioclase, augite, and olivine are 

 scattered. The basalts are more often columnar than the dolerites. They 

 exhibit a marked tendency to become slaggy and amygdaloidal at their 

 external surfaces. The more distinctly crystallized basalts, such as those of 

 Craiglockhart Hill and the Long Row, near Edinburgh, are termed aname- 

 sites by Dr. GEIKIE. The ground-mass of the rocks of the basalt group is 

 ormed of minute lath-shaped plagioclase, granules of augite, magnetite, 

 and a variable quantity of glass. 



Contemporaneous basalt-lavas and intrusive sheets of dolerite are 

 found associated with the Carboniferous strata of the western portion of the 

 Midland Valley of Scotland, and, as would naturally be expected, they bear 

 a very close relation to the rocks just referred to. For a description of 

 some of these rocks we are indebted to Mr. ALLPOB/r/ 1 ) The rock forming 

 the " Hill of Dun," at Bowling, three miles east of Dumbarton, resembles in 

 composition that of the Lion's Haunch, from which it differs merely in 

 the fact that the felspars of the ground-mass are more decidedly lath- 

 shaped, and often exhibit a beautiful arrangement due to fluxion. 



A very interesting variety of basalt is described by Prof. ZmKEL/ 2) from 



(1) British Carboniferous Dolerites, Q.J.G.S., Vol. XXX., 1874, p. 558. 



(2) Basaltgesteine, Bonn, 1870, p. 139. 



