430 PROF. J. W. JUDD ON THE VOLCANIC KOCKS 



canos of Hungary, and a comparative study, by the aid of the 

 microscope, convinced me that these Scottish rocks of Palaeozoic age 

 are only altered forms of andesites, like those of Hungary *. Not 

 only do they agree very closely in ultimate chemical composition, 

 but the structures so characteristic of the andesites — such as the 

 zoned felspars with large glass inclusions, the hornblendes and micas 

 with blackened edges, and the base formed of a- felted mass of micro- 

 lites — are all found exactly repeated in the porphy rites. Further 

 than this we have the groups of the augite-andesites, the enstatite- 

 andesites, the hornblende-andesites, the mica-andesites, and the 

 quartz-andesites of Hungary all exactly represented in Scotland by 

 the augite-porphy rites, the enstatite-porphyrites, the mica-porphy- 

 rites, and the quartz-porphyrites respectively. 



We are indebted to Dr. A. Geikie, I believe, for first pointing out 

 the close agreement between the altered Scottish rocks and those 

 described by continental petrographers under Gustave Rose's and 

 Naumann's name of " porphyrite." Dr. Geikie proposes to class these 

 rocks as '* felspar-magnetite rocks " f, the two minerals named being 

 the only ones which are now recognizable in the altered products. 

 But by examining a series, including varieties in every stage of 

 alteration, the original identity of these rocks with the modern 

 andesites can be very clearly demonstrated. More than this, we 

 are able to detect every stage of the processes by which the trans- 

 formation was effected. 



In the cases of the I^orthfield and Causewayhead rocks, as in those 

 of the Cheviot andesites so well described by Mr. Teall J, there seems 

 to be no valid ground for separating them from the similar rocks of 

 Tertiary age. Studying the rocks in which the first stages of altera- 

 tion appear, we find that it is the ferro-magnesian silicate which is 

 nearly always the first to yield to the agents of change ; and, as a 

 general rule, the porphyritic crystals are altered before the smaller 

 crystals of the ground-mass. The enstatites and augites are espe- 

 cially susceptible to this kind of action, and in rocks showing no other 

 kind of change we find these minerals in a more or less advanced 

 state of decomposition . 



The first stage in the alteration of the pyroxene-andesites consists 

 in the conversion of the pyroxenes into the green decomposition-pro- 

 ducts for which Vogelsang proposed the name of "viridite." This 

 " viridite," which at first forms only pseudomorphs after the original 

 crystals (see Plate XIII. fig. 3), afterwards extends into the sur- 

 rounding glass and eventually fills up all the fissures and cavities of 

 the rock (see Plate XIII. fig. 4). The almost structureless " viridite " 

 breaks up into spherulitic aggregations of various hydrated minerals: 

 but when formed from enstatite these can be shown to be largely 

 composed of serpentine, and when formed from augite to consist 

 mainly of delessite. The alkaline and calcic silicates appear to be 



* Quart. Journ. Greol. Soc. vol. xxxii. (1876) p. 295 ; and ^Volcanoes, what 

 they are and what they teach' (1881) pp. 263-265. 

 t Trans. Roy. Soc. Edinb. vol. xxix. p. 508. 

 + Geol. Mag. dec. ii. vol. x. (1883) pp. 100-109, 145-153, and 254-263. 



