28 H. J. JOHNSTON-LAVIS ON THE GEOLOGY 
transmit a dark burnt-sienna-coloured light, and are probably mag- 
netite, titaniferous possibly, which has undergone alteration. 
Their small size, compared with the large crystals, has rendered 
them more liable to decomposition, especially on the outer surface, 
which accounts for the redder-coloured exterior of the rock-frag- 
ments. The interstitial glass is small in quantity, and of a brownish 
tint. 
The included impurities are exceedingly abundant, enveloped in 
the original plastic mass. 
The intermediate pumice already mentioned as connecting this 
deposit with the underlying one, is variable in colour, but usually 
greenish grey, fading into the tint of the two beds. The larger 
crystals are well terminated, and there are fewer impurities. Some 
of the sanidines are filled with a multitude of small pyroxene crystals. 
The biotite plates are better formed and less broken. The rod-like 
microliths are usually arranged in stars. The microliths of mag- 
netite are less abundant than in the bed above. 
Separating the pumiceous scoria from the bed above, we encounter 
a stratum of very fine dusty material or ash, as in the Vallone 
Breislak. In other sections the same material occurs, but apparently 
partly rearranged by water. 
Puass III., Period 3. 
General Description—The deposits of this period conformably 
overlie those of the last, of which they seem to be the continuation. 
They range from 80 centimetres to 2:30 metres in thickness. 
When seen in section they present a curious aspect due to their 
peculiar characters. The principal portion consists of rounded lapilli, 
variable in size and order of deposition, mixed with a large quantity 
of fine powdery matter of the same kind. 
Three elements go to make up these lapilli. The first and most 
abundant consists of pieces of leucilitic lava, bits of old lava; in fact, 
which have been subjected to very violent attrition, as shown by their 
subangular and often rounded form. The next are of yellow or 
rusty-brown colour, of the same material, but altered by solfataric or 
fumarolic action. The third and rarest element, though most im- 
portant as regards the story it tells, is the primary eruptive matter, 
which consists of an opaque black vesicular scoria, usually in very 
minute morsels. In colour it resembles the scoria below, but, as 
already remarked, is more open in structure. A few crystals of 
pyroxene and black mica may be detected by the unaided eye. 
The whole arrangement of materials is very variable, but frag- 
ments of a given size are usually found on any one horizon; so that 
a band of coarse lapilli may intervene between two dust-beds, and 
so on. 
Microscopical Examination.—The larger constituent crystals are 
the same as in Period 2, but the development of each species has ad- 
vanced much further, and the microliths have increased in size and 
number and are more clearly defined. The greater perfection applies 
