4 8. Allport — On the Pitchstones of Arran. 



formed, exhibiting in section well-defined angles and edges ; many 

 of them are, however, imperfect, and not a few are mere fragments, 

 probably broken by the flowing movement of the mass while still 

 in a viscid state ; but of this motion there is better evidence in certain 

 peculiarities of structure, to be noted presently. 



The felspar crystals frequently contain cavities, many of which 

 are filled with portions of the glassy base and its characteristic 

 contents ; they are generally more or less rectangular in form, and 

 often lie in rows parallel with the sides of the prism. 



Crystals of augite and magnetite are sometimes imbedded in the 

 felspar, it would therefore appear that this mineral was the last to 

 crystallize. 



The quartz is clear and transparent ; it occurs in the sections in 

 hexagonal or rhomboidal forms, and also as short prisms terminated 

 by pyramids. The outlines are generally more or less rounded, 

 sometimes completely so, but very frequently there are one or two 

 well-defined edges and angles, the other sides presenting a peculiar 

 rounded and indented appearance. This is well seen in Figs. 8, 9, 

 and 10, which accurately represent three imperfectly formed crystals 

 containing cavities filled with portions of the base. There are not 

 only cavities inclosed in the quartz, but the indentations frequently 

 open out into channels extending into the interior ; some are pro- 

 portionately much longer than those figured, while others show 

 clearly how many of the cavities were formed. Fig. 12 exhibits an 

 extension of the base into a quartz crystal through a narrow opening 

 in the edge, and it is evident that a slight addition to the quantity 

 of silica would close the aperture and produce a very characteristic 

 form of cavity. Although these cavities vary in size and shape, 

 most of them have a regular rhomboidal form, corresponding with 

 that of the crystal (see Fig. 11) ; and it will be seen that Fig. 12 

 would afford an equally good example if the opening were closed. 



The sides of the longer channels which penetrate the crystals are 

 rounded, and sometimes bent into folds, forming flexures and curves 

 which could only have been produced when the quartz was in a 

 plastic state. It is evident from these facts, that the quartz crystal- 

 lized out from the surrounding matrix, and also that the crystals 

 were at one time small masses of plastic silica, which gradually 

 assumed a crystalline form, and became perfect or imperfect, ac- 

 cording as the conditions were more or less favourable. 



It is important to observe that this mode of occurrence in the 

 pitchstones is equally characteristic of the quartz crystals in the 

 quartziferous porphyries, but that it is quite different from that 

 which obtains in granite. It would appear, therefore, that the con- 

 ditions under which the latter was formed must also have been 

 different, and as the former are unquestionably true eruptive rocks, 

 it may well be that those geologists are not far wrong who hold 

 that in raost cases granite is a metamoi'phic rock. 



The augite has a clear transparent green colour, and occurs in 

 well-formed crystals and irregular grains, but the former are rare ; 

 it is generally imbedded in the base, sometimes in the felspar also. 



