518 A. G. MacGregor and F. R. Ennos — 



a very finely granular purplish deposit, which became dark brown 

 on exposure to bright sunlight. This fine powdery deposit was 

 traceable to the highly corroded patches as a source. By treatment 

 of the slide with aqueous ammonia, all the dark particles were 

 removed. Hence it was concluded that silver chloride had been 

 formed at the expense of some of the isotropic patches, which were 

 thus shown to contain chlorine. 



Two slides of ditroite were then cut from a hand-specimen with 

 blue sodalite apparent to the naked eye, and were similarly treated. 

 The sodalite patches were corroded and became crowded with dark 

 grains, and there was seen on the slides a purple deposit, precisely 

 the same as that already described, which darkened in sunlight. 

 The spreading of this purple deposit from the large sodalite patches 

 was actually observed with a hand lens during the action of the 

 nitric acid and silver nitrate, and it was found impossible to prevent 

 this taking place to some extent. 



In contrast to the phenomena shown by the ditroite, slides of 

 teschenite from the island of Inchcolm and from Crossall Hill, 

 Dalmeny, when similarly treated with nitric acid and silver nitrate, 

 gave no sign of a silver chloride precipitate. Instead, the large 

 analcite patches of these rocks acquired the same reticulated pattern 

 as was observed in some of the isotropic material of Trapraia Law. 

 Very dilute nitric acid and silver nitrate gave the best result. 



A further test for chlorine in the Traprain Law rock was made as 

 follows. A small fragment of the rock was crushed and repeatedly 

 shaken up with cold distilled water. The filtrate did not react for 

 chloride, thus proving water-soluble chlorine was not present in 

 appreciable quantity. Another chip when powdered and treated 

 with cold dilute nitric acid gave a dense precipitate of silver chloride 

 on adding silver nitrate to the filtrate. This proved acid-soluble 

 chlorine was present.^ 



The only sources of acid-soluble chlorine in an igneous rock are 

 the sodalite group of minerals and sometimes apatite. Only very 

 minute apatite needles were present in the five slides cut from the 

 hand-specimen used for crushing, and could not account for the 

 dense silver chloride precipitate, even on the supposition that all 

 the apatite was chlorine-bearing. (Occasionally larger very sporadic 

 apatites are met with in the Traprain Law rock.) Hence part of the 

 isotropic material was referred to a mineral of the sodalite group. 



Because Hillebrand (6, p. 26) states that chlorine is an essential 

 component. of noselite (nosean), another rock chip was powdered 

 and boiled with dilute hydrochloric acid. The filtrate when tested 

 did not react for sulphate. This proved that part of the isotropic 

 material was sodalite. 



The Inchcolm teschenite, when crushed and tested for chloride, 

 gave no appreciable precipitate of silver chloride. This test, when 



^ See also analysis. Part II. 



