Lawson. ] 



Geology of Carmelo Bay. 



31 



The Rock-Forming AT ineral, Iddingsite — In order to establish the 

 specific character of iddingsite it will be convenient to state the 

 results of our observations upon it somewhat in detail, before enter- 

 ing upon the more general description of the carmeloite in which 

 it occurs. Macroscopically iddingsite may be observed in those 

 facies of the carmeloite in which it is best developed to be a mineral 

 having a lamellar structure due to a very facile cleavage. The 

 cleavage lamellae are very brittle and break off sharply in fracture 

 planes transverse to the cleavage. (See Plate 4, Fig. 1.) It scratches 

 selenite with ease, but may be crushed to powder on a cleavage 

 surface of Iceland spar, by rubbing with a soft wooden point, with- 

 out scratching the spar. Its hardness may therefore be placed at 

 about 2.5. Its prevailing color is bronze, and the cleavage faces 

 have a pronounced bronzy metallic luster. Other colors, however, 

 are frequently exhibited, ranging from light brown to deep brown- 

 ish red. Its maximum specific gravity is 2.839 as determined by 

 the aid of Klein's solution; but it varies in this property, and differ- 

 ent lower values may be obtained for different portions of the pow- 

 dered mineral, a fact which renders its separation from the other 

 constituents of the rock a matter of great difficulty. Before the 

 blowpipe it is infusible and is not perceptibly altered. It is very 

 readily decomposed by hydrofluoric acid. When fragments are 

 treated with hydrochloric acid, the application of a little heat imme- 

 diately effects the extraction of the iron, without otherwise decom- 

 posing the mineral or affecting its action on polarized light. Con- 

 tinued digestion of the mineral in the form of powder ends in its 

 complete decomposition with the separation of pulverent silica. It 

 decomposes perhaps even more readily in sulphuric acid, also with 

 the separation of pulverent silica. When heated in the closed tube, 

 the powder yields water, but less abundantly than a similar quantity 

 of powdered serpentine. 



The solutions in hydrochloric and sulphuric acids were exam- 

 ined chemically, after the removal of the pulverent silica by filtration, 

 and abundantly satisfactory reactions were obtained for iron, lime, 

 and magnesia. No aluminum was detected. Moreover the flame 

 test in very numerous experiments showed the constant presence of 

 sodium. Chemically, therefore, iddingsite is a hydrous non-alum- 



