246 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 30, 



2. Basaltic and scoriaceous eruptions, of a sluggish nature (wellings- 

 out), which have caused but little elevation, and no cone. 



3. Cones with cups ; of various compositions. 



1st. Of the tufa-craters there is a greater variety in respect to 

 size (diameter) than in any of the other classes. The Pupuke Lake 

 (No. 2 on the map, PI. XII.) is three-quarters of a mile in diameter 

 between the walls of the crater, while the little Pond-crater, No. 7, 

 is only 30 yards across. 



These craters are generally either filled with water or with a 

 swampy soil, and all show the characteristic section seen in fig. 3. 



Fig. 3. — Diagram of a Tufa-crater. 



Plain. Tufa-crater. Lake or Swamp. Tufa-crater. Plain. 



~Y 



V 



r 



In eight instances the broad tufa-crater contains within it a second 

 point of eruption, constituting a cone, generally isolated, unless con- 

 nected with the margin by the lava-stream which it has emitted. 

 Mount Richmond, No. 25 on the map, is an illustration of this. 



It is worthy of remark that in many cases the tufaceous craters 

 seem, from their copious supply of water, to be fed by springs on 

 which local rains seem to have but little immediate influence. In the 

 case of the crater No. 25, the water is always running, and is of a 

 pure quality. 



The tufa-crater is often nearly filled up by the lava-stream from 

 its central cone, or by the eruptions of some contiguous volcano. 



2nd. The volcanos of this subordinate class are few, or, perhaps, 

 their immediate points of eruption are but rarely apparent, from the 

 circumstance of their being covered by the lava that has flowed out 

 of them. 



Apart from other volcanos, or high above the level of other lava- 

 streams, are large ridges of basalt or scoria?, bearing a surface- 

 ripple, formed during the consolidation of the fused mass. By 

 ripple-mark I here mean such concentric rings or ridges of surface 

 as may be seen on slag that has cooled undisturbed after flowing 

 from the furnace, as shown in fig. 4. 



Pig. 4. — Concentric Markings on the /Surface of a cooled mass of 

 Lava or other molten matter. 



In these cases the molten matter seems to have welled out slowly, 

 without any projectile force or much vaporous explosion. No. 14, 



