218 Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 



parallel^ and often sinuous laminae, which retain a general parallelism of di- 

 rection to those of the unchanged rock. The texture of the alternate stripes 

 varies likewise in the pearlstone, the basis of which consists of a compact 

 substance of pearly lustre, and a deep lead-blue colour, breaking into small 

 angular pieces. This is ribboned by numerous thin zones of black enamel 

 (pitchstone) strongly resembling obsidian, having a complete conchoidal frac- 

 ture, but wanting the acuteness and transparency of the fracture-edges which 

 characterize that mineral. These zones of black pitchstone may be observed 

 to be themselves passing into pearlstone, by inclosing more or less numerous 

 minute spheroids of this substance, identical in colour, texture and lustre, with 

 the broader zones, which consist of an intimate and confused agglomeration 

 of similar globules, and have acquired their semi-lithoidal texture and clearer 

 colour by a devitrijication of the pitchstone. In the part of the bed lying 

 nearest to the prismatic trachyte, by which the change seems to have been ac- 

 complished, the zones are distorted into very irregular and repeated curvatures, 

 and contain large knots and lenticular masses of black glassy pitchstone. 

 This variety of pitchstone is rarely porphyritic. It decomposes superficially, 

 and at the sides of the minute and otherwise imperceptible fissures which in- 

 tersect the rock at right angles to the plane of the laminte, into a compact 

 white substance. Hence wherever the fracture has followed these fissures, 

 the pearlstone appears ribboned with white instead of black stri[)es. This 

 alteration is in some parts not confined to the surface, but affects the whole 

 thickness of the layer of pitchstone, which then resembles a pure white ena- 

 mel, retaining its conchoidal fracture. 



The southern moiety of Palmarola is principally formed of loose stratified 

 conglomerate or tufa. Strata of this nature compose in particular a lofty cliff 

 facing the south-east, and extending from the Capo di Vardella to the Punta 

 di Mezzo-giorno. They everywhere repose conformably on the irregular 

 surfaces of the prismatic trachyte and its accompanying semi-vitreous conglo- 

 merate. They are of the same character as those of Ponza, consisting prin- 

 cipally of triturated pumice, and inclosing, particularly in the lower strata, 

 larger blocks of this substance (the scoriae of the accompanying trachytes), and 

 a few compacter fragments of the neighbouring rocks. Amongst the latter I 

 found some of an ash-gray pearlstone imbedding numerous crystals of felspar; 

 and others of a similar rock in which the felspar is very brilliant and glassy, 

 the spherolitic globules being irregular, generally hollow, and lined with 

 minute crystals apparently of the same substance, while others closely envelop 

 a single imperfect felspar crystal. 



