154 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



and covers its slopes with such "screes" of debris, that had the basalts been poured round it, 

 they must infallibly have had some of its fragments washed down between their 

 successive flows, not a single pebble of it is there to be found. This might not be 

 considered decisive evidence, but it is extended and confirmed by the fact that the acid 

 rock gives off veins which ramify through the basalts. 



In the bed of the south fork of the Scarrisdale stream, a separate boss of granophyre 

 (which under the microscope exhibits in perfection the characteristic structure of this rock), 



"X s - 



\ y,i: y Mm 



Fig. 44. — Section on south side of Cruach T6rr an Lochain, Mull, a, bedded basalts and dolerites ; b, granophyre ; 

 c, felsitic band ; dd, veins of felsite traversing the basic rocks. 



protrudes through the basalts in advance of the main mass, and a little higher up on the 

 outskirts of that mass narrow ribbons of the granophyre run through the basic rocks. 

 The contrast of colour between the pale veins of the intrusive rock and the dark tint of 

 the basalts is well shown in the channel of the water. Similar sections may be seen on 

 the flanks of Beinn Fhada, especially in the great corry north of Ben More, where the 

 granophyre sends a tongue of finer grain between the beds of basalt. On the east side 



of Loch Ba' numerous proofs of similar intrusion 

 may be observed. Thus at the east end of Loch 

 na Dairidh, where the granophyre has been in- 

 truded into the basalts, hand-specimens may be 

 obtained showing the two rocks welded together. 

 On the slopes of Cruach T5rr an Lochain, where 

 the granophyre has a felsitic selvage, the bedded 

 basalts are traversed by veins of the latter material 

 (fig. 44). A little further east, at the head of 

 the Allt na Searmoin, the bedded basalts, some 

 of which are separated by slaggy scoriaceous sur- 

 faces, are intersected by another protrusion from the compact felsitic porphyry (fig. 45).* 

 A mile lower down the same valley a separate mass of granophyre sends out veins into 

 the basalts. 



As the posteriority of the granophyre and felsites to the basalts is thus proved, the 

 further question remains as to their mode of intrusion. Here and there, especially on 



* This rock appears to the eye as a black finely crystalline-granular felsite. Under the microscope " it presents a 

 markedly granulitic structure, consisting mainly of small rounded grains of dirty brown turbid felspar, with isolated 

 granules of colourless quartz. Scattered through the rock, or accumulated in patches, are small spherical or drop-like 

 granules of a bright green augite (coccolite)." — Dr Hatch. 



Fig. 45. — Section at head of Allt na Searrnoin, Mull. 



a, basalts and dolerites, with slaggy upper surfaces ; 



b, felsite. 



