6o8 



KNOWLEDGE & SCIENTIFIC NEWS. 



[December, 1906. 



arc fairlv equal in size, there are many in which they 

 are far from being so, whilst in many, one side may 

 measure twice the size of the smallest side. They vary 

 from about 8 inches to as much as a foot and a half. 

 One of the unexplained phenomena in connection with 

 the pillars is that the horizontal joints which divide 

 them up into lengths of from 6 inches to two or three 

 feet, do not ;dways present strictly a plane surface, but 

 the two portions fit into one another by either a con- 



cavity below and a convexity above, or vice versa. 

 "\\ here the former has been the case, and the upper 

 column has been removed, small pools of water collect 

 in the concave top of the column. 



Where the Causeway proper approaches the cliffs. 



Fig, 4.— Rectangular and hexagonal columns. Note convex surface 

 of hexagonal column In left hand bottom corner. 



and the vertical sides of the columns show themselves 

 eii masse, the weathering of the joints is very noticeable, 

 so that the sharp angles are lost and the different por- 

 tions of the columns have acquired a rounded aspect, 

 and have begun to assume a bomb-like appearance. 

 This is particularly noticeable on the spot whence the 

 illustration (Fig. 6) is taken, situated at the east end 

 of the sixth of the little bays which collectively go to 

 make up the Giant's Causeway, in the broadest use of 



the term. These are not, however, true volcanic 

 bombs, but are merely harder, rounded blocks, from 

 which the matrix in which thev were wrapped has de- 

 cayed. 



The rocks which form the whole of the cliffs are 

 divided into two principal lava-flows, and these are 

 known respectively as the Upper and Lower Basalts. 

 Between the periods represented by these two great 

 flows, suilicient time elapsed to allow of great changes 

 taking place in the upper portion of the earlier flows. 

 This has resulted in the appearance of a great thickness 

 of "red bole," a hydrous silicate of alumina, although 

 thinner bands of the same rock occur between the suc- 

 cessive minor flows, which go to make both the Upper 

 and Lower Basalts. The illustrations (Figs. 3 and 5) 

 show the low'er ends of columns of rather irregular 

 structure, which must have formed deep down in the 

 upper la\ a flow-. They rest upon the broken basalts, in 

 which the fractures took no regular structure, and this 

 in turn rests upon the thick red bole betw-een the two 

 chief flows. Westward from this, the base of the 

 Upper Basalt descends to below sea-level. 



One little promontory which juts out and so divides 

 off the areas of two little hays appears to have resulted 



Fig. 5.— Bomb-like concretions in basalt. 



from having a backbone of harder igneous rock than 

 the rest, and this must have flowed through a crater, or 

 ■' neck," as we should now call it, that had opened out 

 in a fracture in both the Upper and the Lower Basalts. 

 This, indeed, may ha\ e been one of the fissures which 

 fed the supply for the later of the Upper Basalts. How 

 far back mto the land aw-ay from the coast this neck 

 extends has not been determined, but many of the fis- 

 sures, in other places, through which lava has poured 

 extend for many miles. Such a one is the great Cleve- 

 land Dyke, extending across the surface of Yorkshire. 

 They are plentiful in south-west Scotland, and those 

 who know the Biological Station at Millport, Greater 

 Cumbrae, will recognise in the " Lion Rock " one such 

 dyke, and in the great wall on one side of the Station 

 grounds another. Tlie latter can be traced into the 

 cliffs at the back, and under foot, down to the shore in 

 front. It is only about 3 feet wide, but has stood at- 

 m.ospheric denudation to a much greater extent than 

 the Old Red Sandstone rock, into whose fissures the 

 lava welled up. 



Not the least interesting phenomenon in connection 

 with volcanic dykes is the mannei- in which they have 

 affected the strata which they have pierced, and the 



