X)r. Mac Culloch on the Geology of Glen Tilt, 309 



has not hitherto been found in Scotland, and as indeed it seems to 

 have been as yet seen in only one or two situations besides the- 

 place whence its name is derived, it will not be superfluous to 

 describe the specimens and the substance in which they are im- 

 bedded. The base of the porphyry is a brownish hornstone or 

 compact felspar, containing numerous crystals of reddish-yellow 

 felspar and a few grains of quartz, some of which appear to have 

 the primitive form of this substance. Together with these, nume- 

 rous crystals of pinite are imbedded in the stone, varying in diameter 

 from one tenth to one sixth of an inch, sometimes of similar 

 length, and at others mere scales. Brongniart describes it as having 

 been found in porphyries, and I may add that the specimens which 

 I procured differ in no respect from the foreign specimens with 

 which I have compared them. 



A similar porphyry, I may here say, is to be observed in Glen 

 Shee. Oxide of titanium in a pulverulent or scaly and investing 

 form, is also to be seen in the rifts of the quartz rock in Ben 

 Gloe. The same disposition of this mineral occurs in Ben-na- 

 caillich, near Killin, and it must not be confounded with oxide 

 of iron, the more common m-etallic substance in rifted quartz.* 

 Among the loose stones on the skirts of Ben Gloe I also found 

 a singular substance. It is a laminated schist of which the basis is 

 clay slate, occasionally mixed with hornblende and with quartz, 

 and containing small lamellar and conchoidal scales of a crystalline 

 white carbonat of lime. I know not how to describe this rock 

 €0 well as by saying that the calcareous bodies are precisely like 

 fragments of shells, and that the whole on a first view would pass 

 for a shale containing shells ; if found in a secondary country it 



* I may here add that I have also observed rutile in quartz in the Scarsough mountain, 

 and accompanied by oxiduious iron ill the micaceous schist of Rannoch, so that it is aot at 

 iiare mineral in Scotland. 



