178 OBSERVATIONS ON BASALT, &C. 



Inftances: Por- porphyry arc not rare ; and, among other place?, are found 

 phyry, petro- ncaf £) re (y en | ev eral feet in length, and not more than two 



iilex, argilla- ' " 



ceous fchiftus, inches in diameter. Columns of petrofilex compdle a large 



rubble flate, portion of a mountain near Coniftone lake. Very perfect 

 iuneftone, &c. , , . . . ... _ ,.„ r , 



quadrangular prilms of argillaceous lchiftus are round near 



Llanwrft. Rubble (late affumes the columnar form at 

 Barmouth. The Iimeftone near Cyfartha, in Glamorgan- 

 shire, is divided into very regular acute rhomboidal prilms ; 

 even the fandllone of the fame diftrict is not unfrequently 

 columnar ; and one of the beds of gypfum at Montmartre 

 is diftinetly divided into pretty regular columns. Sandftone, 

 clay, argillaceous iron ore, and many other fubftances, 

 become prifmatic by torrefaction ; and the prifms of itdrch 

 formed in drying have often been confidered as illuftrative of 

 bafallic formations. 

 Mere contrac- I am very far from conceiving, that all thele configurations 

 j'° n m ffa°'r are ' n ^ uence< ^ by mcn fyfiematic arrangements as have deter- 

 fembling thefe, mined the form of fome bafaltic columns. I confider mod of 

 but lefs regular, j ^ em as fblely attributable to contraction; which is only a 

 farther extenfion of the aggregative force, and muft be regu- 

 lated by the texture, the form, and the pofition of the mafs. 

 Where the texture of the mafs is homogeneous, and its con- 

 tractions uniform, its dimenfions may be diminifhed, without 

 its continuity being deftroyed, provided its aggregation be fo 

 ftrong as to overcome the vis inertia of the mafs, and its ad- 

 Developement of heiion to other fubfiances. But, when the reliftance is fuffi- 



the effedts of c j e nt to overcome the aggregation, the mafs will be rent by 

 contractor! in „„. ,. , ?,?„....,.' n J 



producing fym- nMures perpendicular to the direction in which the greatefr. re- 

 metric fraftures. fiftance to its contraction takes place, or, in other words, by 

 Allures perpendicular to its greateft furface : for it is from the 

 extremities of the greateft furface, that the largeft quantity of 



perfect prifms ; but the moil beautiful I have feen, are the fin all 

 ones from Ponza. The columns at Boli'ena are faid to be bafalt. 

 Thofe of the Euganean hills are very irregular in form; in their 

 texture they are certainly wholly unlike granite, which Mr. 

 Strange thought they it ten i hied. I believe them to be lava. 



The mention of iorne columnar formations that follows, is by 

 no means intended as an enumeration of them. I have confined 

 myfelf to thofe which I have either infpected in their natural fitua- 

 tion, or of which I have (een numerous fpecimens. 



4 matter 



