BY LEO A. COTTON. 237 



(3) The considerable depth of alluvial in the lake, probably 

 representing a filling-up of the crater. 



(4) The inner ridge at the south-west of the lake, which pro- 

 bably is a remnant of an inner crater so common in volcanoes. 



(5) The great height of the lagoon, from which the land falls 

 away in all directions (See PI. xxi., fig.2), save immediately to the 

 south. This rise to the south is due to the more active source of 

 eruption at Black Mountain. 



Conclusion. — In view of the evidence above given, it seems 

 probable that many other mountain-lakes in this district, situated 

 at high levels, such as the Ben Lomond Lagoon, are of similar 

 origin. The identification of these lakes as centres of volcanic 

 eruption, will be useful in solving such problems as the sequence 

 of the lava-flows, the periods of maximum and minimum volcanic 

 activity, and the location of deep leads. 



EXPLANATION OF PLATE XXL 



Fig.l.— Guyra Lagoon, looking towards the east. The rising ground near 

 the central part of the "photo is the inner crater shown as A in 

 text-fig. 1. 



Fig.2. — Section through the Guyra Lagoon from north-west to south-east, 

 showing the geological structure of the district. 



i' -:^ 



