December 1, 1898.] 



KNOWLEDGE. 



267 



north-westerly and south-easterly direction. The great 

 Cleveland dyke, which cuts the Jurassic strata of Yorkshire, 

 must be included among them ; and outlying members 

 occur about Lough Erne, and even in the County of 

 Galway. Sir A. Geikie" estimates that the "dyke-region 

 embraces an area of upwards of forty thousand square 

 miles — that is, a territory greater than either Scotland or 

 Ireland, and equal to more than a third of the total land 

 surface of the British Isles." This, however, is but a small 

 matter, compared with the whole region involved in the 

 volcanic activity of early Cainozoic times. Suffice it that, 

 as a detail in the general overtlow, the downs of Antrim 

 and Londonderry became buried in successive lava-flows. 



Even the advocates of " fissure-eruptions," as a means 

 of flooding a whole province with lava, now regard the 

 molten rock as flowing from a number of points along the 

 track, each centre resembling an ordinary volcanic vent. 

 The flows coalesce in the hollows, mount upon their 

 predecessors, spread now this way, now that, and eventually 



Fig. 1. — Columnar Basaltio Lava-llow, resting upon old iand-siirface of denuded Chalk. 

 QiiariT at Whitehead, Belfast Lough. Typical Sectiou iu the Antrim Plateaux. The 

 lava has been subsequently denuded, and boulder-clay has been deposited across the whole. 

 Photographed by Mr. R. Welch. 



obliterate all the features of the landscape. New vents 

 may break through this rudely stratified accumulation, and 

 may sometimes build up true scoria-cones on the surface, 

 as their action becomes more irregular and explosive. A 

 country deluged with lava from small " puys, ' like those 

 on the central plateau of France, may finally come to 

 possess a few isolated volcanic mountains, from which the 

 last products are ejected. \A'hen all dies down, when 

 denudation works its will, the separated cones are all but 

 swept away. Perhaps their mere necks, filled with 

 crystalline lava or with coarse agglomerate, remain 

 standing out above the earlier fields of lava. Then the 

 latter become cut into by the streams ; the buried land- 

 scapes are in places restored to light ; while the masses 

 left between the newly cut valleys have the form of table- 

 lands and plateaux, capped by the relics of the flows. 

 In our northern volcanic area these successive events can 



* Work quoted, Vol. II., p. 121. 



be followed out. While many of the dykes never reached 

 the surface, others may easily have been responsible for 

 the basaltic flows. Olivine-basalts and basaltic andesites, 

 sometimes retaining a glassy structure in their ground- 

 work, sometimes of almost doleritic texture, cover the 

 irregular surface of the chalk. Their lower portions have 

 often become columnar, where they contracted on cooling 

 in contact with the loose flint gravels (Fig. 1). The 

 separate lava-streams can be traced out in the great cliff- 

 sections, and are seen to dovetail into one another, each 

 great basaltic " stratum " being formed of several adjacent 

 and overlapping flows. Steam-bubbles, globular in form, 

 or elongated by the flow of the molten mass, or strikingly 

 irregular, are everywhere in evidence, especially near the 

 surfaces of the flows. In many cases, white nests of 

 zeolites, chalcedony, or opal, have formed within them, and 

 probably began to fill up the cavities as soon as the lavas 

 came to rest. Often the upper part of a flow is rubbly and 

 irregular, while the lower part, which cooled more slowly, 

 has assumed a bold colum- 

 nar structure, so that the 

 flow appears at a distance 

 to consist of two distinct 

 types of lava. This feature 

 is conspicuous on the bold 

 headlands round the 

 Giant's Causeway ; and 

 the Causeway itself is the 

 basal portion of a stream 

 of similar character. 



The Giant's Causeway 

 owes its fame to the ex- 

 quisite regularity of its 

 columnar structure, and to 

 the neatness of its curving 

 cross - joints. The east 

 coast of Skye, or the cliffs 

 of Loeh-na-Keal, in Mull, 

 may produce nobler vol- 

 canic landscapes ; while 

 the isolated relic of a mas- 

 sive lava-flow. now forming 

 the Isle of Staffa, is far 

 more wild and picturesque. 

 But the district of the 

 Giant's Causeway mustal- 

 ways remain as a perfect 

 museum for the student, 

 and the black dykes that 

 jut out into the water are 

 as characteristic in the landscape as the flows themselves. 

 The majority of the dykes that are revealed in the fine 

 series of sections along the scarps of Antrim cut through 

 both the chalk and the lower lava-flows. They form 

 black and often sinuous bands, traversing the quarry -faces ; 

 and their mere abundance is in the highest degree impres- 

 sive. The lava-flows that may have been connected with 

 their rise have often been entirely swept away. But we 

 have clear evidence that pauses occurred in the activity at 

 various points, for the lava-sheets that remain are often 

 separated by bands of red earth, which are the products of 

 the weathering of one flow before the next was poured out 

 across its surface. These layers, well known also among 

 the black chffs of Skye, form striking Unes of colour in 

 the sections. The broad red band, running along the 

 cliffs east of the Causeway, cannot fail to strike every 

 visitor, and points to a time of general rest throughout 

 the district. By its means, as exposed here and at other 

 places, the eruptive series has been divided into two stages ; 



