591 
tins indicates a ratber mild type of volcanic energy, in wbicli the eruptions were not 
highly explosive hut only moderately so. The more violent eruptions are attended with 
the ejection of great quantities of fragmental products, while the milder ones disgorge 
little else than flowing lava. It is well known that the greater the amount of frag- 
mental ejecta, the steeper and more compact is the cone or mountain built up around 
the vent, while the fluent lava spreads out over wide areas, oftep flowing great distances, 
and the slopes which it generates are much gentler and sometimes imperceptible to the 
unaided eye. As already remarked, fragmental ejecta are common here, and I have seen 
none at ail away from Mount Taylor except the lapiili in the cinder cones. We shall 
appreciate this mode of growth of a great lava-cap when we come to study the modern 
lavas in the San Jose Valley, where their freshness enables us to see every detail of the 
process. 
“We may infer, then, that the state of affairs in the region now occupied by the 
valley of the Puerco was, during the activity of the vents now represented by the necks, 
much the same as that which is indicated in the mesa above. Prom 1,000 to l,-i00 
feet (according to locality) of Cretaceous strata, since eroded, then overspread the 
valley and regions to the eastward and southward, also to the northward, from which 
they have been swept away. Over their surfaces the lavas were outpoured from many 
vents. The eruptions were of a ‘mild’ type, attended with little violence, and the ejecta 
were doubtless lava with few fragmental products. These streams, issuing from many 
vents, became interwoven with one another, and through a long period of time accumu- 
lated, sheet upon sheet, to great thickness, just as they have done in the valley of the 
San Jose in modern times. The result was a lava-cap differing in no respect from that 
which is now seen upon the surviving mesas.’’ 
NEWER VOLCANIC SERIES. 
The grounds on which I separate the Post-Mesozoic Volcanic rocks into a Lower 
and Upper Series are (1) that the former was laid down after the elevation of the 
Desert Sandstone, but before it had suffered any considerable amount of denudation, 
while the latter was laid down after the former had suffered denudation, in some cases 
sufficient to allow of the Desert Sandstone and even of underlying formations being 
exposed ; and (2) that since the deposition of the former, most, if not all, of the vents 
from which the lava was poured out have been removed by denudation, while the vents 
of the latter series are still extant, either as crateriform hollows, crater-lakes, or 
“ necks,” filled with plugs of basalt or ash. ^ i 
To the latter (the Upper) Series belong the basaltic rocks north of Cooktown- 
Basalt has emanated for the most part from volcanic centres, which occur generally in 
the form of dome-shaped unwooded eminences near the heads of the valleys which have 
been denuded out of the sandstone tablelands. Conspicuous among these are the 
“ Sisters,” at the head of the Endeavour, the “ Piebald Mountain,” Mount Morgan 
(Cooktown), &c. These hills do not present a crateriform appearance, but are mere 
rises, marking the site of the lava eruption, which has spread around them when situated 
on level ground, or escaped in glacier-like coulees down the valleys. These points o 
eruption bear, in fact, such relation to the lava-flows as the similar in uvergne 
hear to the basalt there. CouHes of basaltic lava have flowed, from t e foci a ove 
referred to, down the Valleys of the North and South Forks of the Endeavour River, and 
have radiated out from Mount Morgan and other centres to the east and north, oyer the 
flats between the mountains and the sea, where they form by their decomposition a 
chocolate-coloured soil of great depth, peculiarly fitted for agriculture. 
