170 



ACTION OF BASA'LT ON ADJACENT ROCKS. 



basa'lt are often melted on the surface, quartz and feldspar are 

 cracked, sometimes enveloped or penetrated by vitreous matter. 

 Marls, earthy limestones in contact with basa'lt, or pierced by its 

 veins, and especially fragments of matter drawn into the basaltic 

 mass, are converted into compact limestone, sometimes approach- 

 ing the saccharoid state. These limestones also become magne- 

 sian, and are converted into true dolomites, distinguished from the 

 rest of the enveloping mass by their slow effervescence. Dolomi- 

 sa'tion seems to be due to the presence of igneous products. When 

 basaltic veins pass through carbona'ceous deposits, the clays are 

 calcined, the coal is deprived of its bitu'men, and assumes a baccil- 

 hr (berry-like) structure. 



Basa'ltic deposits, in tables, hillocks, or veins, are more abundant on the 

 surface of the globe than all the lavas in ascertained currents, which is, 

 doubtlessly, owing to their mode of ejection. Basa'lts are found in France, 

 on the borders of the Rhine, in Saxony, Bohemia, &c. Iceland contains a 

 great quantity, and the same rocks predominate in the West Indies, at St. 

 Helena, &c., and in almost all the islands of the South Seas. 



Basa'ltic formations are noticed wherever they occur, in consequence of 

 the tendency of the principal rocks to divide into long prisms, the varied 

 arrangements of which have often excited the admiration of the curious. 

 Here all the prisms cdifrerge at the summit of a hillock ; there they form 

 magnificent colonnades of the most picturesque appearance ; in another 

 place all the columns, broken at the same level, present a pavement com 

 posed of pieces regularly joined, extending over a greater or less space, and 

 sometimes formed into an amphitheatre, one above the other. The gran- 

 deur, the imposing appearance of these pavements, have obtained for them 

 the name of Giants 1 Causeway. 



The Giants' Causeway in Ireland is famous ; but a similar structure 

 exists in France. Sometimes there are excavations in the middle of ba- 

 ea'ltic masses, or trappean rocks, which resemble them most, some of them 

 forming very remarkable grottoes. The most celebrated is Fingal's cave, 



in the island of Staffa, 

 which is formed in the 

 midst of trap, divided 

 into prismatic columns 

 with the utmost regu- 

 larity, and into which 

 the sea continually beats. 

 Others exist in the ba- 

 sa'lt, properly so called ; 

 there is a famous one 

 on the banks of the 

 Rhine, between Treves 

 and Coblentz, near Ber- 

 trich-Buden (Jig. 281), 

 the columns of which 

 are composed of rounded 

 pieces, which has caused 

 Fig. 281. Cheese-grotto, at Bertrich-Bfiden. tnem to be compared to 



files of cheeses, whence the name of cheese-grotto, common in the country 



9. What influence does basa'lt exert over adjacent rocks ? What is 

 meant by dolornisation ? Give some instances of basaltic formation. 



