370 TROF. J. W. JUDD ON THE PROrVLITES 



the close connexion that always appears to exist in different vol- 

 canic regions between solfataric action and eruptions of rock of acid 

 composition *. The same fact has also been pointed out by Schmidt t. 

 While the ejection of basaltic lavas is followed by actions that lead 

 to the formation of carbonates, the extrusion of great masses of highly 

 silicated materials is attended and followed by the escape of steam 

 containing sulphurous and other gases, which give rise to the 

 phenomena of solfataric action. 



The rocks of the Western Isles of Scotland afford a very striking 

 illustration of this connexion between solfataric action and the 

 ejection of highly silicated rocks. 



B. Contact-MetamorjjJiism. 



Studied microscopically, the sheaths of altered " felstone " that 

 surround the intrusion of granite or gabbro enable us to understand 

 the succession of changes which place as the result of the contact 

 of these rocks with great bodies of fused materials. As already 

 pointed out, these effects are strikingly contrasted with those re- 

 sulting from solfataric action. 



We are, in the first place, forcibly reminded of the resorption- 

 halos which are seen surrounding hornblendes, micas, and other 

 minerals as the result of the action of a heated magma upon them. 

 But this action, instead of being confined to the immediate proximity 

 of the crystals afi'ected, may extend to the distance of some inches or 

 even feet from the planes of contact. 



All the ferro-magnesian silicates — pyroxenes, hornblendes, and 

 micas — break up into finely granular aggregates, which seem to 

 eonsist of an almost colourless pyroxene and of magnetite grains, 

 though other minerals may not improbably be present. In some 

 cases, however, the very minute granules appear to have the colour, 

 pleochroism, and other optical properties of melilite (see Plate XIV. 

 fig. 8). 



In the midst of these granular aggregates we sometimes find 

 scales of a deep brown, highly pleochroic biotite making their ap- 

 pearance ; and these increase in size and in number as the igneous 

 mass is approached. Clear colourless needles can also be detected, 

 and these may not improbably be referred to some species of secon- 

 dary felspar. 



In these greatly altered rocks it is only possible to state what 

 was the nature of the original rock, by tracing the alterations step 

 by step from the comparatively unchanged mass at a distance from 

 the intrusions right up to the planes of contact. 



YIII. Light thrown by the Study of these Tertiary Lavas on 

 SOME OF THE Older Yolcanic Rocks (Porphyrites, Pelstones, 

 &c.). 



It was pointed out, in my former memoir, that in Scotland we 

 have brought close together remarkable masses of volcanic materials 



^ Bull. Soc. Geol. Fr. ser. 3, vol. xvii. pp. 282-290. 

 i" Zeitscb. d. deutsch. geol. Gresellscb. 1885, p. 737. 



