Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/117

 but in a few instances some of the smaller druses were found in what may be considered to be their original condition. They are then filled with a soft impalpable powder consisting of carbonate of lime, often containing a considerable number of small but perfectly formed crystals of celestine ; these are not attached to the rock, but are loose in the powdery matrix. The form is in all cases the well- known combination Poo. 1/2 Poo. OP .oo P, characteristic of the celestine of Girgenti in Sicily. The angles correspond perfectly with those given in Dana's Mineralogy. The twin planes are oo Poo for parallel, and oo P for intersecting macles. The latter are often of considerable size, forming St. Andrew's crosses from 2 to 3 inches in length. The smaller and more perfect crystals are generally without included fossils. On comparing a great number of crystals of both kinds, it becomes easy to trace the progress of the decomposition, which invariably begins by a roughening of the faces of the prism, owing to the formation of a number of fine grooves or striations parallel to the basal cleavage. The action of the solvent goes on in the same way, enlarging the grooves until a great part of the interior of the crystal is eaten away, leaving a funnel-shaped cavity where the prism- faces formerly existed ; and this may continue until the whole of the original form has been removed. In proof of this may be mentioned the occurrence of a limestone-pseudomorph after celestine : only a single specimen was found ; and the crystal replaced was so rough that it was somewhat difficult to make out at first ; but when broken, the comparative sharpness of the basal plane was at once apparent. More generally, however, the transformation stops short of a complete change, the crystals being rough and hollowed at the ends and sides, but keeping their freshness, and even in some cases their colour, on the basal planes. The partially destroyed crystals then form a basis for a secondary growth of the same mineral ; but these new crystals are remarkable for their brilliancy and perfection ; the form is the regular Girgenti combination, but they are generally somewhat squarer and thicker, and are free from included fossils. The largest of these secondary crystals measured about 3 inches in length and 1-1/2 inch between the basal planes, and is perhaps the largest known example of this form, the American crystals from Strontian Island on Lake Erie, though larger, being of a different character.

The fact of the occurrence of fossils in the older celestine crystals is sufficient to show that they are of synchronous origin, or that the crystals have been formed by gradual deposition from a tolerably concentrated solution, such as might be imagined to exist in a sea whose bed was diminishing from the evaporation of the water. The hollows in the limestone may be compared to small basins where the crystallization took place, any fragments of shells, small Nummulites, and Bryozoa (either living or dead, but probably the latter) that happened to be present in the calcareous mud having been entangled up and enclosed. Forchhammer* obtained sulphate of strontia direct by evaporating sea-water, and Wackenroder, in 1836†, pub-

† Ann. Chem. und Pharm. vol. xli. p. 316.
 * Bischof, vol. i. p. 449 (2nd edition).