individual rock-masses usually exhibit a uniformity in the character of the 

 porphyritic constituents ; in other words we do not find the different types 

 just referred to occurring together in one and the same rock-mass. Olivine 

 sometimes occurs in sufficiently large crystals to give a porphyritic character 

 to some of the basalts. 



The presence of accessory minerals, such as olivine, enstatite (hyper- 

 sthene) mica and hornblende, gives rise to additional varieties. We thus have 

 olivine-dolerites, enstatite-dolerites, mica-dolerites and hornblende-dolerites. 

 The occurrence of two or more of the accessory constituents gives rise to 

 still more complicated forms. 



So far we have been referring only to original characters. Tf now we 

 take into consideration secondary characters, due to the operation of the 

 various agents of metamorphism,the subject becomes stillfurther complicated. 

 Chlorite, zeolites, calcite, quartz and other minerals are developed by surface 

 agencies. The members of the dolerite group associated with the older strata, 

 and especially those of lower Palaeozoic age, frequently contain chlorite. 

 These constitute the typical diabases. Contact and regional metamorphism 

 also produce important structural and mineralogical changes. Hence arise 

 proterobases, epidiorites, and various schistose rocks. 



A special feature of contact metamorphism is the development of a pale 

 brown mica; and, less frequently, of such minerals as tourmaline, axinite 

 and garnet. 



It thus appears that members of the great dolerite-family vary in respect 

 of the foUowing characters : 



(1) Size of the individual constituents. 



(2) Chemical composition. 



(3) Mineralogical composition. 



(4) Amount of glass or other interstitial matter. 



(5) Relation of the pyroxene and felspar as regards the order of formation. 



(6) Presence or absence of porphyritic constituents and the character of the porphyritic 

 constituents when any are present. 



(7) Extent and character of the metamorphism. 



Owing to the large number of more or less independent variables the 

 group is extremely rich in varieties. Some of these are of no special 

 significance ; others have a chronological, geographical, or geological value. 

 To ascertain the relative value of the different characters is one of the most 

 important problems awaiting solution. 



In describing the members of the dolerite group and the changes to 

 which they have been subjected it will be convenient to follow the plan 

 adopted in other parts of the work and speak of them as they are developed 

 in the different districts in which detailed observations have been made. 



West of Scotland. The Tertiary dolerites and basalts of the west 

 of Scotland, and the north of Ireland first demand our attention. They 

 form beds of great extent and thickness Avhich must have flowed at the 

 surface as lavas. They occur also as veins, dykes, and intrusive sheets. 

 The medium grained rocks, or dolerites proper, are for the most part 

 holocrystalline compounds of plagioclase giving lath-shaped sections, augite 

 in irregular crystalline masses, magnetite or ilmenite, and olivine. They are 

 in short, typical ophitic olivine-dolerites (see Fig. 1, Plate X.). Good illustra- 



