The Geology of Cowal. 89 



Hornblende-porpliyrites and felsites are classed together, since the 

 same porphyritio constituents, viz., felspar, black mica or chlorite, 

 sometimes hornblende, and scattered quartz blebs, occur in both. 

 These rocks behave in the field in the same way as the lamprophyres 

 (presently to be mentioned), generally forming sheets which run 

 roughly with the foliation of the schists ; they are of limited extent. 

 An intrusive boss of some importance is also described as hyperite, 

 the rock consisting of hypersthene, augite (diallagic in part), 

 plagioclase (more or less lath- shaped), iron-ores, and interstitial quartz. 



Of the older igneous rocks are those described under the terms 

 lamprophyre and mica - trap. It is only when the mica is 

 macroscopically prominent that the lattex term seems applicable. 

 By a gradual decrease in the amount of mica the mica-dolerites may 

 pass into rocks of more normal doleritic aspect. In all these early 

 dolerites which have been examined the augite differs from that 

 of the Tertiary basalts in belonging to the pale form, malacolite, and 

 for the most part occurring porphyritically. In the north and east 

 of Cowal tliese lamprophyres are exceedingly numerous, but they 

 are not known within a distance of four or five miles of the Upper 

 Old Red Sandstone boundary. They rarely form vertical dykes ; 

 when they do so, these dykes usually run in a difi'erent direction 

 to that of the basalts. Most frequently they occur as sheets with 

 varying and not very steep hades to the horizon. They do not keep 

 to the bedding or foliation of the schists, but may be constantly seen 

 cutting across their crumplings. It is evident that all the move- 

 ments in connection with the foliation of the schist had ceased before 

 their intrusion. Though thin and inconstant, the lamprophyres are 

 of considerable interest in working out the geological structure of 

 the district, and a greater help than the basalts. This is because the 

 majority of the faults ai-e of later date than the lamprophyres, and 

 throw them, so that they help to indicate the difi'erent faults and even 

 the amount of their respective throws, independently of the schists. 



An inspection of the geological map will show that parts of Cowal 

 are seamed by basaltic dykes : this subject is very fully treated in 

 the Memoir. Basalts, dolerites, and tachylites constitute the group, 

 with which even augite-andesites are included. Thin margins or 

 selvages of distinctly glassy rock, tachylite, are common, but not to 

 be found without close search. One of the most curious features of 

 basaltic dykes, especially noticeable on the coast, is the tendency for 

 some to weather in relief, whilst others form recesses sometimes 

 hollowed out into caves. The authors consider that the size of the 

 grain is often a determining factor in these cases. The intimate 

 structure of the basaltic rocks is treated of at considerable length. 



With respect to the relative age of the dykes, the broad east-and- 

 west dykes are regarded as older than the basalts having a north- 

 west direction. " If we suppose the early east-aud-west dykes 

 to be of Carboniferous age, the interval of time between them and 

 the north-west dykes must be immense; for there can be no doubt 

 that many of the latter belong to the same set as those which, still 

 with a north-west direction, are seen in the island of Mull to intersect 



