194 



Dunglass, in Strathblane, ten miles N.N.W. of Glasgow. It is composed 

 of lath-shaped felspar, altered olivine and augite in a glassy globulitic 

 base. The globulites are extremely minute yellowish brown or brown 

 bodies, the largest of which are only < 004mm. in diameter. Where these 

 bodies are very numerous the glass appears a dark brown. They are 

 without action on polarized light. Prof. ZIRKEL remarks that the poverty 

 of this rock in augite is compensated for by the abundance of the brown 

 globulites in the base. 



Near Skilliangow, on the south slope of the Campsie Fells, occurs a 

 very beautiful porphyritic mica-bearing dolerite (or basalt). The augite is 

 well crystallized, but contains numerous inclusions of magnetite and felspar. 

 Owing to the abundance of the porphyritic constituents the rock has a 

 coarsely crystalline aspect. Mr. ALLPORT has also described rocks from 

 Neilston-side Hill, the Craig of Neilston, and Croft Head, near Neilston, on 

 the south side of the Clyde, in Kenfrewshire. The rock of the Craig of 

 Neilston is an exceptionally good illustration of a basalt largely consisting 

 of minute columnar crystals of augite in a felspathic matrix. 



Among the intrusive sheets of the Clyde Basin Coal-field that of the 

 Necropolis Hill, Glasgow, deserves special mention. It is a micaceous 

 dolerite of the Car Craig type (see ante p. 191). The rock of Deniston Hill 

 is similar in character, but contains veins in which augite and horn- 

 blende (basaltic hornblende) are present (see Fig. 1, Plate XIII.). Hornblende 

 is occasionally found in the rock of the Necropolis Hill. The amygdaloidal 

 dolerite of Mugdock Tunnel, near Glasgow, furnished Mr. ALLPORT with a 

 fine series of microscopic preparations illustrating the changes which 

 igneous matter of basic composition is subject to. "The least-altered 

 portions of the compact dark coloured varieties show that the rock consists 

 of augite, plagioclase, magnetite, and serpentinous pseudomorphs after 

 olivine, together with much disseminated chlorite, some of the augite and 

 felspar being but slightly altered. In other specimens the whole of the 

 constituents are converted into pseudomorphs, still retaining the original 

 forms of the crystals, and there are many small cavities filled with 

 calcite, zeolites and vermicular chlorite. These compact varieties, having 

 a micro-amygdaloidal structure, afford extremely interesting examples 

 of alteration, and illustrate in a most satisfactory way the gradual 

 progress of the various changes which have taken place." 



It seems probable that the cavities referred to are in part, at any 

 rate, not true amygdaloids but pseudo-amygdaloids, that is spaces deter- 

 mined by the decomposition of minerals " in situ," and filled with secondary 

 compounds resulting from the interaction of the chemical substances 

 which pass into solution under the influence of the percolating waters. 



The southern portion of the Island of Arran is extremely rich 

 in rocks of the group now under consideration. It contains also 

 rocks which may be conveniently designated andesitic dolerites (or 

 basalts) in order to indicate their relations to the rocks of inter- 

 mediate composition. By some authors these rocks would be termed 

 augite-andesites. It is a matter of indifference what we call them 



