346 PEOF. J. ^y. jtjdd on the propylites 



Though often very dark-coloured, their lustre is usually dull, and 

 they seldom if ever exhibit the jet-black tint and velvety aspect so- 

 often found in the olivine basalts of the district. The rocks are 

 usually, though by no means invariably, of porphyritic structure, and 

 the crystals of felspar have the opacity and the absence of vitreous 

 lustre so often seen in the more ancient and plutonic rock-masses. 

 Not unfrequently rocks that appear at first sight to be perfectly 

 compact are shown hj their mode of weathering to have been 

 originally made up of angular or rounded fragments, a conclusion 

 which is confirmed by microscopic study. Under the microscope, 

 indeed, many compact and seemingl}' homogeneous rocks are seen to 

 present all the characters of volcanic agglomerates and tuflfs, and are 

 found to be composed of a great variety of ejected fragments. 



On freshly fractured surfaces or on faces that have been weathered, 

 these rocks often exhibit evidence of possessing a strikingly banded 

 and fluidal structure, and, under the same conditions, the porphy- 

 ritic habit which is so general in these rocks becomes very con- 

 spicuous. It is very remarkable to find that many rocks which 

 are of a uniform dull grey colour, and apparently quite homogeneous 

 or compact in texture, must originally have presented all the charac- 

 ters which distinguish flowing lava-streams or beds of scoriaB and 

 lapilli. These conclusions as to the original characters of the vol- 

 canic materials are fully confirmed, as we shall see when we come to 

 study their microscopical characters. It is evident that the altera- 

 tion, attended in many cases with an almost complete recrystal- 

 lization of their materials, has effectually masked their original 

 characters. 



Many of these rocks were evidently originally scoriaceous ; and, as 

 the result of weathering, these often assume at the surface their 

 pristine slaggy appearance. In many cases the cavities are occujDied 

 by aggregates of epidote- (pistacite) crystals usually enveloped in 

 zeolites ; and these clusters of green crystals, by the weathering-out 

 of the surrounding zeolites, stand up in relief on the exposed surfaces 

 of the rocks. Yeins and nests of epidote also abound in many of 

 these rocks. In other cases the black enclosures (originally green) 

 to which Macculloch gave the name of Chloro2)haeite are very con- 

 spicuous. 



Titreous varieties of these rocks sometimes occur, and such per- 

 fectly glassy parts of the rocks often exhibit but little alteration. 

 The same fact is illustrated in the andesites of the Cheviot Hills, 

 where the glassy portions arc far less altered than the associated 

 stony portions of the same mass. It would seem that these glassy 

 varieties resist the percolation of solvents through their mass to a 

 much greater extent than rocks made up of aggregates of dissimilar 

 particles, and thus remain in a comparatively unaltered state. 



The white crust which covers the weathered surfaces of the Scot- 

 tish propylites has been already referred to as one of their most dis- 

 tinguishing characteristics. The origin of this white crust appears to 

 he as follows : — by changes, which will be hereafter described, 

 much of the iron has separated as secondary magnetite. Almost 



