263

sulphur as porphyry, the Fontainbleausandstone and burrstone as greywacke,gypsum as serpentine. Mechanical actionseems to have commenced at a very earlyperiod, and chemical to have continued upto the formation of the latest strata.

Dolomieu a was, I believe, the first whomade the conglomerate, or old red sand-stone, the line of demarcation between theprimitive rocks and the secondary.

The structure of rocks changes, by almostimperceptible gradation, from granular tocompact, from compact to porphyritic, fromporphyritic to amygdaloidal, &c. In gene-ral, however, the crystalline rocks may besaid to be the oldest; the sandy, marly,clayey, the newest.

The granular varieties of limestone are,for the most part, older than the compact,and the compact older than the earthy.

Slate occurs in all formations; but themore perfect varieties seem confined to theprimitive and transition rocks.

1 Journal de Physique, vol. xxxix. p. 390.

’ s 4