140 MR J. D. FALCONER ON THE 



structure is commonest. Where enclosed in felspar or in mesostasis the augite shows 

 idiomorphic outlines, but where it encloses felspar it is allotriomorphic and ophitic. 

 Frequently the same crystal may be in part idiomorphic and in part ophitic, the peculiar 

 habit being due to the more or less simultaneous crystallisation of felspar and augite.* 

 In the centra] portions of the sills the augite assumes a long columnar habit, rarely 

 idiomorphic in the prism zone. (See PI. II. fig. 2.) Smaller felspars are occasionally 

 enclosed in an ophitic manner on the margins, but as a rule the felspathic material has 

 crystallised later and moulded the augite. The later quartz has also sometimes exerted 

 a corrosive action upon the augite where not protected by being enclosed in felspar. It 

 is thus very probable that the general absence of definite outlines where such were to 

 be expected is due to the extensive corrosion which the augite has suffered after 

 crystallisation. These columnar crystals have also frequently suffered a mechanical 

 deformation,''' being bent into various curving forms, and often broken transversely 

 when the limit of elasticity has been passed. (See PI. II. fig. 3.) Almost any 

 section from Carribber quarry will show this peculiar structure. The long columnar 

 felspars, however, so rarely show any trace of bending that the phenomenon must be 

 ascribed to some movement within the igneous mass soon after the augite had crystallised 

 out. Most probably the central framework of columnar augites, which may be supposed 

 to have been formed within the igneous magma, was, on reduction of the volume of the 

 mass, unable to withstand the pressure of the overlying rocks, and in consequence 

 collapsed, the individual augites being in many cases bent, shattered, and broken. In the 

 lighter coloured portions of the larger sills the augite frequently assumes, macroscopically, 

 a long curving and branched prismatic form with notable idiomorphism, the various 

 branches from the same stem being arranged in a fan-like manner in the same plane. 

 Similar occurrences, though microscopic, are sometimes found in the diabase aphanites 

 and in their ocelli. (See PI. III. fig. 1.) It is difficult to account for this peculiar habit. 

 It certainly cannot be considered the result of resistance to growth in a viscous medium 

 (as feathery crystals usually are), for, apart from the notable idiomorphism of the fibres, 

 the augite, which is always of the purplish brown type, was here one of the first 

 minerals to crystallise, and is usually found enclosed in the columnar felspars. 

 Most probably the peculiar habit is due to the poverty of certain portions of the 

 magma in calcium and magnesium. 



The shape of this idiomorphic augite is noteworthy. As a rule the pinacoids are 

 greatly developed at the expense of the prism faces ; consequently transverse sections, 

 both of the stout idiomorphic prisms and of the finer branching crystals, assume outlines 

 which are much nearer those of rhombic pyroxene than of ordinary basaltic augite. 

 This introduces a certain element of uncertainty into the recognition of serpentinous and 

 chloritic pseudomorphs of these minerals. 



The augite crystals, both ophitic and columnar, are frequently twinned on the ortho- 



* Cf. "Sub-opliitic" structure, Watts in Geikie's Ancient Volcanoes of Great Britain, vol. i. p. 147. 

 t R08ENBU8CH, Mikro. 1'hys., vol. ii. p. 130. 



