44 
TYPES OF VOLCANOES 
BOOK I 
They form wide level or gently undulating tablelands, which rise in bold 
escarpments from the surrounding coimtry and have been deeply carved 
into valleys. The sides of their clihs and slopes are marked by parallel 
lines of terrace, arising from the outcrop of successive sheets of lava (kigs. 
11, :165). 
With the Tertiary basalt-plateaux are connected thousands of dykes, that 
follow each other along nearly parallel lines in a general north-westerly direc- 
tion, and mark the position of fissures up which the molten lava ascended. 
Occasional necks of agglomerate or basalt indicate the sites of some of the 
eruptive vents. 
The Carboniferous volcanic plateaux have been more extensively denuded 
than tliose of Tertiary age, so that a large number of their vents have been 
laid bare. In general these vents are of comparatively small size, though 
larger than those of the Carboniferous Puys. In some districts, abundant 
dykes traverse the rocks on which the plateaux rest, though the fissures 
seem to have been less numerous than in Tertiary time. 
a. The Puy type, as before remarked, takes its name from the well- 
known or volcanic cones, of Central Prance. Volcanoes of this type 
form conical hills, generally of small size, consisting usually of fragmental 
materials, sometimes of lava. Where a cone is partially effaced by a second, 
and even by a third, successive slight shiftings of the vent are to be inferred 
(see Pigs. 29 and 214). In many cases, no lava has issued from such cones, 
nor were the ashes and cinders dispersed far from the vent. Hence, in the 
' progress of denudation, cones of this kind are easily eflaced. 
Prom the uniformity of composition of their materials, the simplicity and 
regularity of their forms, and their small size, it may be inferred that many 
of these cones were the products of single eruptions. They may conceivably 
have been thrown up in a few days, or even in a single day. The history of 
Monte Nuovo, in the Bay of Naples, which was formed within twenty-four 
hours in the year 1538, is a memorable example of the rapidity with which 
a cone more than 400 feet high may be thrown up at some distance from a 
central vent. 
The smallest independent volcanoes are included in the Puy type. In 
many instances the diameter of the funnel has not exceeded a few yards : 
the largest examples of the type seldom exceed 1000 feet in width. 
Where lavas have been discharged, as well as ashes and stones, a more 
vigorous activity is indicated than where merely cones of tuff were formed. 
The lavas may come from more than one side of a cone, &nd may flow in 
opposite directions for a distance of several miles. It is observable that 
considerable streams of lava have issued from the base of a cinder-cone 
without disturbing it. The molten rock has found a passage between the 
loose materials and the surface on which they I'est,^ though, in some cases, 
the cone may have been thrown up after the emission of the lava. 
In the history of a puy there is commonly a first discharge of fragmentary 
material ; then lava may flow out, followed by a final discharge of loose stones 
1 M. Boule, Bull. Carle Gcol. France, Ifo. 28, tome iv. p. 232. 
