679 PHILOSOPHICAL TRANSACTIONS. [anNO 1775, 



tium ad partes. In fact, these crystals manifest no such porosity. The columns 

 of Achon, though of a homogeneous substance, yet differ from the common 

 basaltes by their immense size as well as colour, which is rather brown than 

 black. - The columns of San Biasio are likewise very large, measuring often 2 

 feet in diameter. They are also of the simple species, that is not jointed, and 

 mostly quadrangular, which figure seems rather a principal characteristic of this 

 group, being rarely observed in others. So true it is, as I formerly remarked, 

 that some particular characteristic ever distinguishes the different groups of 

 basaltes ; which therefore cannot be too narrowly observed, before we pretend to 

 form any opinion about their origin. Some few, but very few, chiefly of the 

 smaller columns of San Biasio, are of a pentagonal form. But there are no 

 hexagonal columns, which, in other basaltic groups, are the most common. The 

 natural position of these columns, whether facing the rock, or about the 

 bottom of it, is mostly perpendicular. 



Another adjacent portion of this rock is also characterized by angular, and as 

 it were winding strata, somewhat resembling the bending pillars of StafFa. The 

 rock itself is also composed of angular masses, as are indeed most granites ; and 

 these masses are also ranged perpendicularly. Several emerge, as it were, from 

 the tops and sides of the neighbouring rocks and hills, like so many stately and 

 artificial pillars. The winding strata before-mentioned are also parallel with each 

 other, as frequently observed in other granites, as well as common vulcanic 

 strata in general, particularly of the harder sort. Desmarest calls the latter 

 Basaltes en tables ; which is a kind of vulcanic slate, formed in parallel strata of 

 different thickness, from 2 or 3 to 5 and 6 inches. This is very common in the 

 provinces of Velay and Auvergne, in France, where it is also used for coverings 

 of houses. The same sort of slate is likewise common to the mountains of 

 G^enoa, many of which seem to be of vulcanic origin. These slaty tables, or 

 parallel strata, of granite, are observed near the top of the famous San Gothard, 

 in the ascent of that mountain on the side towards Switzerland. These strata 

 are also ranged perpendicularly, like the other common ones in granites, and 

 resemble Desmarest's basaltes en tables ; affording thus another proof of the 

 analogy remarkable between the organization of the different masses in granites, 

 and that of common vulcanic strata in general. The former, as well as the 

 latter, have their prismatic columns, their basaltes en tables, as Desmarest calls 

 them, and en boules. Surely therefore these are strong proofs in favour of the 

 common origin of both. The rocks of San Biasio abound with ferruginous 

 vitrifications, which are frequently observable in granites ; and the neighbouring 

 tracts with lava or pori ignei. 



XLII. Observations on the Difference between the Duration of Human Life in 



