Leonard Hawkes—Tridymite in Icelandic Rocks. 207 
retention of the lamellar form of the tridymite. In those now to be 
described the outline of the quartz is not that of the tridymite 
lamelle, being more or less rounded. ‘This type is well illustrated 
in a fragment 1 mm. in diameter, composed of twenty-three variously 
sized quartz grains, embedded in the Faskrudsfjord rock. Fig. 5 is 
a microphotograph of a part of the fragment, and Fig. 6 shows its 
outline and that of its component individuals. The quartzes are 
allotriomorphic with respect to one another, and a rapid glance 
conveys the impression of the usual granitic texture. On closer 
investigation, however, they are seen to have straight lines running 
Fic. 6.—A drawing of the aggregate of Fig. 5. The dotted lines are the traces 
of the former tridymite lamelle. 
through them, commonly in pairs, giving a rod-like appearance. Some 
of these lines are very distinct with a medium magnification, and they 
can be seen in Fig. 5, but many of them are only discernible using 
a high power and after continued observation. The lines discovered 
are dotted in in Fig. 6 ; none could be found in some of the quartzes. 
The structure is seen on slight movement of the microscope tube. 
The lines are dark and accompanied by ‘ white lines’, those forming 
a ‘pair’ or ‘rod’ deflecting in the same horizontal direction on 
movement of the tube. The lines obviously represent planes or 
cracks, each pair having a common inclination to the plane of section. 
