344 



PROFESSOR HEDDLE ON 



The specimens all lie loose, being the most enduring portions of veins which have 

 themselves endured after the disintegration of a very micaceous gneiss. To all appear- 

 ance, indeed, the specimens seem to represent the " branches" or knots upon very thin 

 veins of quartz, which veins can, after considerable search, be seen formed in the rock, 

 and such as I have found were barren of minerals. The loose lying fragments of veins 

 consist of a melange of quartz, andalusite, labradorite, fibrolite, and an ill-defmed 

 black mica.* These minerals interlace in a confused manner, there being no approach 

 to a uniformity in growth from the two sides of the vein. The andalusite crystals, 

 indeed, sometimes pass from side to side, lacing the other ingredients together. The 

 mineral is always rudely crystalline, but regular crystals are very rare. The most 

 perfect I have delineated. 



Though I have figured them as " complete " in the terminal planes, yet all the 

 crystals I have seen had these planes hemihedrally disposed. The colour is a uniform 

 dull purplish red ; but there is this most important fact to be noted, that all the 

 crystals which can be sectioned and examined, though uniform in structure and trans- 

 parent in thin slices, have a central core which is deep purple, with purple spots at the 

 four corners of the transverse section, after the manner of chiastolite. Well-crystallis<'d 

 andalusite thus seems to have a complex internal structure which is independent of any 

 portion of the matrix being caught up during its concretion into a geometric solid. 

 This fact, not, so far as I know, before noticed, comes to have an important bearing in 

 all speculations as to the question of the mode of formation of chiastolite crystals in 

 clay-slate rocks. 



The crystals are sometimes 3 or 4 inches in length, and occasionally an inch in 

 thickness. Rarely, as noticed by Sowerby, they form a tube-like sheath to a central 

 core of the felspar ; there is not here the slightest appearance of any passage into 

 felspar, as assumed by Bournon ; but there is an almost insensible passage into, or 

 intermixture between it and colourless and brilliant lustred fibrolite. 



The specific gravity of this red andalusite is 3 "121. 



The analysis on 1/302 grammes yielded — 



Silica, 



. -442 



from Alumina, 



. -036 





•478 = 36-712 



Alumina, 



. 59-678 



Ferric Oxide, . 



2-302 



Manganous Oxide, . 



•230 



Lime, .... 



•860 



Magnesia, 



trace 



Water, .... 



•465 



100-247 

 Insoluble silica, 4-184 per cent. 



* See Chapter V. The Micas (Trans. R.S.E., xxix. (1879) 33). 



