1905-1906.] 339 



that it was a secondary mineral, and classed it as a member of 

 the chlorito-ferruginous group or green earths. As far back as 

 the year 1837, however, Dr. Scouler described it as a 'pitch- 

 stone,' or volcanic glass, and in 1843 General Portlock called 

 it an 'obsidian.' It is of interest to trace the history of geo- 

 logical opinion concerning hullite from these earliest references 

 to those of the present day. In the years 1837-1843, it was 

 regarded as a volcanic glass, which had been exuded into the 

 cavities as the lava cooled. About the year 1868 this opinion 

 was upheld by G. V. du Noyer, but ten years later Hardman 

 departed entirely from this view, as we have already stated, 

 describing it as a secondary mineral of the green earth variety. 

 In 1879,, as Professor Cole has pointed out, Dr. Heddle, of St. 

 Andrew's, supported Hardman in his claim for the retention of 

 hullite as a definite mineral species. It is here worthy of 

 notice, that Heddle's views concerning the origin of such 

 'green earths' underwent a complete change; in 187 1 he re- 

 garded such minerals as contemporaneous with the last stages of 

 cooling in the early history of the lavas in which they are 

 found, whereas in his "Mineralogy of Scotland," written at a 

 much later date, he describes the same minerals as decomposi- 

 tion products of the weathering lava. In the same year (1879) 

 William Gault, of Belfast, took up this idea, that hullite was a 

 'secondary' mineral, and accounted for its origin by supposing 

 that it was deposited, along with chalcedony and other siliceous 

 minerals,, from hot alkaline springs which arose in the volcanic 

 neck 'long after the volcanic forces had spent their vigour.' 

 The hot alkaline water acted on the rock sides and penetrated 

 the ground of the rock, dissolving mineral matter from the 

 mass, and re-depositing it in the gas-vesicles and veins. In 1885 

 hullite was again examined, on this occasion by Lacroix, who 

 also determined it to be a secondary mineral related to the 

 decomposition-products of olivine. Latterly, in 1895, Professor 

 Cole reviewed the opinions of Hardman, Hull, Heddle, and 

 Lacroix, and, setting aside the idea that hullite is a 'secondary 

 mineral' at all, described it as 'a basic glass that has become 

 soft and "gummy' by alteration.' 



