ORIGIN OF BASALT. 28^ 



We have another variety of basalt, whose surfaces external. Facts a^d ob- 

 and, ifl be allowed the expre- Ion, internal, are all curves : its servauons re- 

 r ■ 1 ... , r -1 Ti specting basalt 



lorm IS round, and it is composed oi concentric spheres, like in the county 



the pellicles of an onion. 'J^ Antrim ; ad- 



This variety Mr. St. Fond himself admits not to be of vol- that it is not 

 canic origin. He says (Min. des Vole, page 46), it must have volcanic, 

 takeii this configuration naturally. Its mode of arrangement, 

 in the places where it i^ found, seems still more extraordinary. 

 It is geneialiy imbedded in an indurated basaltic paste ; in Mr. 

 St. Fond's language, incorporie et incastrSe dans des massifs 

 dc hasalte injor':ic. In this state, it is sometimes built in the 

 form of a wall, of which the globular basalt is the stones, and 

 the unformed the cement. 



I have great reason to believe, that the varieties of basalt 

 in -other countries are exactly the same as in our own ; and 

 that nature has taken the same pains to keep them distinct 

 everywhere. 



The columnar basalt, of all countries, corresponds precisely 

 with that of the Giant's Causeway, and our other groups, 

 as appears from the sameness of their curious articulations. 



Our irregular prismatic exactly answers the description of 



vex bases add two more ; but, by Sir Torbern Bergman's definition, 

 crystals are bounded hy plain surfaces. 



These facts cannot be cxl.ibited in distinct joints; for the cohesion 

 is so strong, that the ascending pyramids invariably break off, as the 

 joints are separated from the pillar. It is the projecting fracture that 

 remains, which gives the joint the appearance of a mural crozcn, as was 

 observed by the early writers on the subject. 



The destruction of these ascending pyramids makes the sepa- 

 rate joint totally different from what it was, when existing in the 

 perfect pillar. 



To illustrate all this, I give a drawing of two pillars : one, as it ap- 

 pears when long exposed to the air, which acts principally upon the 

 joints; while the dilation and contraction, from heat and cold, loosens 

 the pyramids, and separates them from the pillar. 



The second pillar exactly represents the state in which they appear, 

 where the mass is lately quarried iniOj and the air has not had time to 

 operate.' 



I add some joints in their natural state. This nicety of constructior\ 

 abates, as the pillars graduate through imperfection to an amorphous 

 mass-; yet occasional traces of it are long observable. 



VoL.XVI.— April, |807. 2 A the 



