362 
DES. T. ANDERSON AND J. S. FLETT ON THE ERUPTIONS OF THE 
flowing streams. Such alternations give a pronounced stratification to these accumu¬ 
lations, yet the individual beds are never persistent but always very local, and the 
irregidarities of the bedding and of the dip are explained by the rapid variation of 
tropical streams in volume and velocity of flow. 
Although lava, flows are not the predominant feature of the coast sections, and are 
far less imjjortant than the ash beds—yet it is not to be imagined that they are rare 
or absent. There are many fine examples of them on both the leeward and the wind¬ 
ward shores of St. Vincent. Their mass is often considerable, as they are frequently 
40 feet and sometimes 80 or 100 feet thick, and some are nearly a mile in length. 
They are mostly andesites or andesitic basalts, porphyritic, with large crystals of 
pyroxene, plagioclase felspar, and commonly olivine. On the fresh fracture their dark 
colour and vitreous lustre often indicates the presence of considerable amounts of a 
glassy base. 
The thickness of these lava flows and the large area they cover are two of the most 
striking features of the geology of the island. Among the more important examples 
may l)e mentioned the lava which outcrops on the south side of Kingstown Bay, that 
which occurs below Petit Bordel, at Chateaubelair, and that which forms the Black 
Point, south of Georgetown. On the leeward coast, near Cumberland, and also near 
Barrualli, thick streams of lava may be seen in the sides of the valleys a little distance 
inland. By the vertical clifis they form they can be easily traced as they run up the 
valleys, and it is clear that they are of considerable age ; for, as they appear on both 
sides of the stream at the same level, they were at first continuous, and have been 
cut uj) into separate blocks as the streams deepened their course and worked through 
them into the softer ashes below. It is this alternation of ash beds with columnar- 
jointed lavas which yields under the influence of sub-aerial erosion those picturesque 
and varied effects which make the island of St. Vincent famous for the beauty of its 
scenery. 
It is possible that a certain number of those crystalline rock masses may reall}^ be 
not lavas which flowed out on the surface, but massive intrusive sheets injected 
between the bedding planes of the ashes; and in that tropical climate, amid the 
dense forests, and on such difficult ground, it would be a task of no small labour to 
establish in every case what is their true nature. Wherever it was possible to 
investigate thoroughly their structural relationships, they proved, with very few 
exceptions, to be true lava flows, though they are not frequently scoriaceous, even on 
their upper surfaces. The bedding of the ashes in which they lie is, as already 
stated, rough and irregular, so that it is not easy to show that they are strictly 
interbedded or conformable. Small discrepancies between the apjjarent bedding of 
the tuffs and the lava flows are not to be regarded as important. On the whole the 
greater lavas form flat sheets with a gentle uniform dip which agrees with that of the 
tuffs where these exhibit a good bedding. Owing to their greater regularity in this 
respect, the lavas afford better evidence to the geologist as to the true dip and 
