THE NOIOIETALLIC MINEBALS. 259 



up into rhombohedral forms, with smooth, lustrous faces, and (2) its 

 doubly refracting property, which is such that when looked through 

 in the direction of either cleavage surfaces it gives a double image. 

 (Specimen No. 53673, U.S.N.M.) It is to this property, accompanied 

 with its transparency, that the mineral, as a crystallized compound, 

 owes its chief value, though as a constituent of the rock limestone it 

 is applied to a great variety of industrial purposes. When not suffi- 

 ciently transparent for observing its doubly refracting properties the 

 mineral is readily distinguished by its hardness (3 of Dana's Scale) and 

 its easy solubility, with brisk effervescence, in cold -dilute acid. This 

 last is likewise a characteristic of aragonite, from which it can be dis- 

 tinguished by its lower specific gravity (2.65 to 2. 75) and its cleavage. 

 Calcium carbonate, owing to its ready solubility in terrestrial waters, 

 is one of the most common and widely disseminated of compounds. 

 Only the form known as double spar, or Iceland spar, need here be 

 considered. 



Origin and mode of occurrence. Calc spar is invariably a secondary 

 mineral occurring as a deposit from solution in cracks, pockets, and 

 crevices in rocks of all kinds and all ages. The variety used for 

 optical purposes differs from the rhombohedral cleavage masses found 

 in innumerable localities only in its transparency and freedom from 

 flaws and impurities (Specimen No. 53673, U.S.N.M.). The chief 

 commercial source of the mineral has for many years been Iceland, 

 whence has arisen the term Iceland spar, so often applied. For the 

 account of the occurrences of the mineral at this locality, as given 

 below, we are indebted mainly to Th. Thoroddsen. 1 The quarry is 

 described as situated on an evenly sloping mountain side at Reydar- 

 fjorden, about 100 meters above the level of the ocean and a little east 

 of the Helgustadir farm. (See Plate 12.) 



The veins of spar are in basalt and at this spot have been laid bare 

 through the erosive action of a small stream called the "Silfurlakur," 

 the Icelandic name of the spar being " Silf urberg. " The quarry open- 

 ing is on the western side of this brook, and at date of writing was 

 some 72 feet long by 36 feet wide (see fig. 1). In the bottom and 

 sides of this opening the calc-spar is to be seen in the form of numer- 

 ous interlocking, veins, ramifying through the basalt in every direction 

 and of very irregular length and width, the veins pinching out or 

 opening up very abruptly. In fig. 2 of plate is shown an area of 

 some 40 square feet of the basaltic wall rock, illustrating this feature 

 of the occurrence. Fig. 3 of the same plate shows the largest and 

 most conspicuous vein, the smaller having been omitted in the sketch. 

 The high cliff's on the north side of the quarry are poorer in calc-spar 

 veins, the largest dipping underneath at an angle of about 40. 



1 Geologiska Foreningens I, Stockholm Forhandlingar, XII, 1890, pp. 247-254. 



