60 



fissureb existed throughout the area, out of which the lava was 

 obtruded in limited quantities. The field exhibits an ever- 

 varying surface of rough ridges and bosses of lava, which rise 

 from 12 to 20 ft. above the normal level. In a few instances 

 a stream has flowed for a short distance, but, for the most 

 part, the lava has formed as an excrescence over the fissure 

 from which it exuded, and the feeble attempts at flow have been 

 circumscribed by other lava ridges which existed in close proxi- 

 mity. 



A very instructive example of the way in which these lava 

 ridges and bosses have been formed can be seen in an isolated 

 ridge, which evidently owes its existence to a fissure on the 

 north-west side of the mountain, originating at the base of the 

 cone, and extending for about a quarter of a mile in a direc- 

 tion at right angles to the Mount. The lava is in the form 

 of a steep broken ridge, which reaches a maximum height of 

 about 40 ft. There is no sign of a stream or lateral outflow, 

 but the igneous material has penetrated the superincumbent 

 ashbeds in the form of intrusive veins, and thrown the beds in 

 places into a high angle of dip. In consequence of this dis- 

 placement of the superficial beds of ash the latter supplied 

 the lateral walls by which the igneous rock was confined, where 

 it imperfectly broke through the surface. The fissure has 

 apparently greatly varied in its size at different points of its 

 length. This can be noted by the broken outline of the ridge, 

 dying out almost to nothing at one spot, and at others in a 

 high angle of outcrop, forming an igneous boss that looks like 

 a great basaltic wall. 



AVhere the road crosses this ridge, which it does in one of its 

 gaps, some excavation has been carried out and revealed a 

 most interesting section, showing the relationship of the lava 

 to the older ash beds. Tongues of lava can be seen penetrating 

 the ashbeds, distorting them, but do not reach the surface. 

 Others, somewhat stronger, break through the superincumbent 

 mass and reach the surface. The section now described forms 

 the bank on the east side of the road. On the west side ci 

 the same cutting the ashbeds are practically undisturbed, but 

 within a few yards they are raised at a high angle, and the 

 lava of the ridge reaches its greatest altitude in the height 

 already mentioned. The ridge is separated by a considerable 

 distance from the main lava field; the intervening space is 

 level, and occupied by ash or soil. 



A question of considerable interest is involved in the rela- 

 tion which the lava deposits bear to the volcanic ash with 

 respect to time. Did the lava eruptions precede the ejectment 

 of ash which built up the cone around the central vent? Or 

 did such eruptions occur concurrently with the showers of ash ? 



