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Geological Society. 



ture, from holocrystalline forms (diorites) through various "grano- 

 phyric" and " pilotaxitic" types into true vitreous rocks ("pitch- 

 stones "). They are found constituting lava-streams, which are 

 usually short and bulky ; eruptive bosses or " Quellkuppen ; " and 

 lenticular intrusions or " laccolites." 



By carefully following in the field the much-altered rocks to points 

 where they retain some of their original characters, the propylites 

 can be shown to represent various interesting types of andesite and 

 diorite. The amphibolic and micaceous rocks include hornblende- 

 andesites, hornblende- and mica-andesites containing enstatite, 

 diorites, and quartz-diorites. Among the pyroxenic rocks the most 

 noticeable varieties are the labradorite-andesites, the pyroxene- 

 andesites — of which both " trachytoid " and " vitrophyric " forms 

 occur, — as well as examples of what have been called " dial- 

 lage-andesites " (the nature of the pyroxene in which was fully dis- 

 cussed) ; these rocks are found passing into augite-diorites. and 

 quartz-augite-diori tes . 



A microscopic study of these rocks enables us to investigate the 

 processes by which they have acquired their peculiar characters. The 

 chief and most widely operating cause of change is thus demonstrated 

 to have been solfataric action ; and this was shown to have accom- 

 panied the intrusion into the andesites of masses of igneous material 

 of highly acid composition (granites and felsites). This solfataric 

 action has been developed around each of the five volcanic centres 

 described in 1874. A smaller and much more local cause of change 

 — some of the results of which are strikingly contrasted with those 

 of the wide-spread solfataric action — is found in the contact -meta- 

 inorphism resulting from the intrusion into the andesites of masses 

 of igneous rock of basic, intermediate, or acid composition. 



The much- altered propylites of Tertiary age were shown to have 

 their exact analogues among the older (Palaeozoic) lavas of Scotland 

 and other districts. These rocks, which have been called " fel- 

 stones," " porphyrites," &c, are andesitic lavas, some of which have 

 suffered only from the action of surface-waters, while others among 

 them must have been profoundly affected by solfataric action and 

 converted into propylites, prior to the operation of the surface- 

 agencies of change. 



Forming a very striking contrast with the older Tertiary andesites 

 (propylites), are the numerous scattered and generally small masses 

 of rock which belong to a late period in the Tertiary, and constitute 

 the youngest volcanic rocks of the British Islands. These rocks are 

 found intersecting, in the form of dykes, the great sheets of olivine- 

 basalt ; or where poured out at the surface (as at the Sgiirr of Eigg 

 and Bemn Hiant in Ardnamurchan), lie upon their greatly eroded 

 surfaces. The rocks in question, which have the mineralogical 

 constitution of augite-andesites, are remarkable for the wide 

 variations in their aspect and chemical composition ; and this 

 was shown to result from differences in the proportion of the crys- 



