3 86 • CLAY GENUS. Obfervations. 1. It has been very often confounded with pumice, from which it ’differs very much, as may be feen by comparing the defcriptions given in this work. 2. Its particular geognoftic chara&ers vrill be given at full length in the Geognofie; at prefent I fhall only mention one character, firft obferved by Wer ner, which fufficiently didinguifhes it from unaltered rocks. When lava contains cryftals of hornblende, augite, or Ieuzite, they are wrapped up, not imbed ded in its bafis; and, when they occur in veficles or air holes, one part of the cryftal proje&s into the cavity, and the other is included in the lava, and does not therefore, as is the cafe with zeolite or other fofiils formed by infiltration, &c. fill the air holes, form drufes, or only cover the fides. 3. It is the opinion of many mineralogies that lava fhould not find a place in a fyftem of Ory&°', gnofie, but for what realon 1 have never been able to learn. Werner confiders it as entitled to its prefent place, bCcaufe it is mechanically fimple. . ' Uje. O11 account of its lightnefs it is employed f° r building, particularly for arching vaults. The rock eut of which the ferrous millftoncs of Andernach cut
