BASALT. 



BASALT. 



391 



True Basalt has been regarded as composed of Augite, Felspar, and 

 oxide of ijon ; but this definition is far too limited for either theoreti- 

 cal or practical purposes, unless the constituent minerals be considered 

 of variable chemical compositions, as appears to be the case. Since 

 Augite and Hornblende may, from the researches of Rose, be regarded 

 as the same mineral, it follows that a very fine-grained Greenstone, 

 containing a considerable per-centage of oxide of iron, can, even under 

 this definition, be considered a true Basalt. There can indeed be little 

 doubt that the same igneous rock has been termed Greenstone when 

 the grains of Felspar and Hornblende were sufficiently distinct, which, 

 when exceedingly fine-grained, has been named Basalt. Basalt can 

 only be considered as one variety of that mass of melted rock which 

 has been ejected at various periods from beneath the crust of the 

 globe, and to which various names have been assigned, according to 

 the characters which circumstances have impressed upon different 

 portions of it. 



Like others of the same class, Basalt occasionally passes into many 

 rocks which have been in a state of fusion beneath the surface of the 

 earth, and subsequently ejected. Dr. Hibbert notices a passage of 

 Basalt into Granite in the Shetland Islands. (Brewster's ' Edinburgh 

 Journal of Science,' vol. i. p. 107.) When however we view the mass 

 of igneous rocks generally, it appears that Basalts are the products of 

 comparatively late geological epochs. We may therefore infer that 

 during the earlier states of our planet, conditions were not favourable 

 to their production, or at least to their propulsion to the surface ; 

 though probably some varieties of Hornblende Rock, particularly when 

 impregnated with much oxide of iron, do not differ materially from 

 Basalt in their chemical contents. The mode of occurrence of these 

 rocks and of Basalts is however very different. 



Basalt is a rock of very extensive occurrence on the surface of the 

 earth, and is very frequently detected in the vicinity of volcanoes, 

 both extinct and active. The greatest mass of Basalt yet observed is 

 that noticed by Colonel Sykes in the Deccan, constituting the surface 

 of many thousand square miles of that part of India. This immense 

 mass of Basalt is either massive, prismatic, or globular, occurs in hori- 

 zontal beds, and is traversed by dykes [DYKE] of Basalt, which some- 

 times cross each other. There is no trace of any crater in this 

 basaltic region ; and indeed this is the case with numerous other 

 districts of Basalt, whence it has been inferred that such tabular masses 

 have not been ejected from a conical vent similar to those of volcanoes, 

 but that the Basalt of which they are formed rose through cracks and 

 fissuies while in a highly liquid state, spreading out in sheets of 

 melted matter over the adjacent rocks. 



As Basalt is frequently columnar, it is a rock which has excited 

 much popular attention, and 

 travellers have been some- 

 times induced to describe 

 rocks as basaltic merely be- 

 cause they were columnar, 

 which however is a character 

 that this rock possesses in 

 common with many others of 

 igneous origin. When Basalt 

 occurs in horizontal tabular 

 masses, and is columnar, the 

 columns are generally peq>en- 

 dicular, aa in the annexed 

 figure. When Basalt forms 

 the substance of a perpendi- 

 cular dyke, cutting through 

 other rocks, and is columnar, 

 the cofumns are usually hori- 

 zontal, in the manner repre- 

 sented beneath, a being the 

 basaltic dyke, and ft ft the rocks 

 through which the dyke passes. 

 Basaltic columns are some- 

 times also curved, and of this 

 mode of occurrence there is a 

 beautiful example in the island 

 of Staffa. __- 



When basaltic columns are jointed, and exposed to the destructive 

 action of breakers on a coast, they often present the appearance of 

 some great ruined 



work of art. Such 

 deceptive appear- 

 ances are however 

 not confined to 

 coasts, for in some 





countries, and es- 

 pecially in India, 

 masses of Basalt 

 rise suddenly from 

 tli.- |j|;iiii, and the 

 broken columns, 



nig upwards, may readily at a distance be mistaken for 

 buildings. When viewed from above, the heads of a number of 

 basaltic mns, if unbroken, appear like a pavement composed 



of numerous polygonal pieces of stone fitted into each other, as in the 

 following figure. 



According to Mr.' Gregory Watt the 

 columnar structure of Basalt is due to 

 the pressure of numerous spheres or 

 spheroids on each other during the cool- 

 ing of the rock, such spheres or sphe- 

 roids being produced in planes of refri- 

 geration or absorption. This author 

 took seven cwts. of an amorphous Basalt 

 named Rowley Rag, kept it in fusion 

 for more than six hours, and cooled it so gradually that eight days 

 elapsed before it was taken from the furnace. The shape of the mass 

 was uneven, and while the thinner portion was, in consequence of 

 more rapid cooling, vitreous, the thicker was stony, the one state 

 passing into the other. It was observed that numerous spheroids had 

 been formed, sometimes two inches in diameter. They were radiated 

 with distinct fibres, the latter also forming concentric coats when 

 circumstances were favourable to such an arrangement. When the 

 temperature had been sufficiently continued, the centres of the 

 spheroids became compact before they attained the diameter of half 

 an inch. When " two spheroids came into contact no penetration 

 ensued, but the two bodies became mutually compressed and sepa- 

 rated by a plane, well defined, and invested with a rusty colour," and 

 when several met they formed prisms. 



The following are Mr. Gregory Watt's inferences from these facts : 

 " In a stratum composed of an indefinite number in superficial extent, 

 but only one in height, of impenetrable spheroids, with nearly equi- 

 distant centres, if their peripheries should come in contact in the same 

 plane, it seems obvious that their mutual action would form them into 

 hexagons ; and if these were resisted below, and there was no opposing 

 cause above them, it seems equally clear that they would extend their 

 dimensions upwards, and thus form hexagonal prisms, whose length 

 might be indefinitely greater than their diameters. The farther the 

 extremities of the radii were removed from the centre, the greater 

 would be their approach to parallelism ; and the structure would be 

 finally propagated by nearly parallel fibres, still keeping within the 

 limits of the hexagonal prism with which their incipient formation 

 commenced ; and the prisms might thus shoot to an indefinite length 

 into the undisturbed central mass of the fluid, till their structure 

 was deranged by the superior influence of a counteracting cause." 

 (' Observations on Basalt, &c. ;' ' Phil. Trans.,' 1804.) 



According to this theory, which is certainly the best hitherto 

 framed to account for the columnar structure of Basalt, the irregu- 

 larity of the prisms would 

 obviously depend upon the un- 

 equal distances of the centres 

 of the spheroids, and the con- 

 sequent unequal pressure ; and 

 it is further inferred that the 

 joints sometimes observable 

 in basaltic columns correspond 

 with the concentric coats 

 noticed above. Two of the 

 most beautiful examples of 

 columnar Basalt hitherto dis- 

 covered are found in the British 

 Islands,one forming the Giant's 

 Causeway, on the north coast 

 of Ireland, and the other 

 at Staffa, among the Hebrides. 

 The largest columns yet ob- 

 served are found at Fairhead 

 at the former place, where, 

 according to the accurate 

 measurement of some by the 

 Ordnance Trigonometrical Sur- 

 vey of Ireland, they are 317 

 feet in height, the sides of 

 these enormous prisms occa- 

 sionally measuring 5 feet. 



Some non-columnar Basalts present no trace of any particular 

 arrangement of parts, while others show a globular structure, so that 

 when the rock becomes more decomposed it has the appearance of 

 numerous bombshells and cannon-balls cemented together by a ferru- 

 ginous substance. This globular structure is sometimes also apparent 

 when the decomposition of the rock has not been considerable, being 

 well exhibited in the concentric arrangement of coats of Basalt round 

 centres at variable distances from each other. 



Other Basalts are amygdaloidal, containing a variety of substances, 

 such as Agates, Onyxes, and other minerals, which have been infil- 

 trated into cavities formed by bubbles of gas or vapour while the rock 

 was in a state of fusion. As these bubbles have sometimes been 

 lengthened by the flow of the rock before it finally cooled, the infil- 

 trated contents filling such lengthened cavities have the appearance of 

 almonds sticking in the mass of the rock, whence the name amygda- 

 loid. When, as sometimes occurs, a great tabular mass of Basalt is 

 composed of superimposed beds, some columnar, some amorphous, 



