1868.] BATJEKMAlf AJTD FOSTER CELESTINE. . 43 



but in a few instances some of the smaller druses were found in 

 what may be considered to be their origiDal 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 .|Poo . OP .00 P, characteristic of the celestine 

 of Girgenti in Sicily. The angles correspond perfectly with those 

 given in Dana's Mineralogy. The twin planes are cx) Poo for parallel, 

 and 00 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| 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 dej^osition 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 hoUows in the limestone may be compared to small basins where 

 the crystallization took place, any fragments of shells, small ISTum- 

 mulites, and Bryozoa (either living or dead, but probably the latter) 

 that happened to be present in the calcareous mud having been entan- 

 gled up and enclosed. Porchhammer* obtained sulphate of strontia 

 direct by evaporating sea-water, and Wackenroder, in 1836 f, pub- 



* Bischof, vol. i. p. 449 (2nd edition). 



t Ann. Chem. und Pharm. vol. xli. p. 316. 



