32 University of California. [Vol. i. 



inous silicate, of iron, lime, magnesia, and soda. The easy extrac- 

 tion of iron by acids without decomposition of the mineral indicates 

 that a considerable proportion of that element is present, not as 

 part of the silicate molecule, but as a pigment in the form of hema- 

 tite or Hmonite, probably the latter. 



The form of the crystals of iddingsite, as seen in hand specimens 

 of the carmeloi'te with the aid of a lens, is found very generally to 

 be conditioned by: (1) The cleavage representing one pinacoid; (2) 

 two pair of prismatic (dome) faces at right angles to the cleavage, and 

 intersecting in an angle, the average value of which for several 

 rough measurements is about 8o°; and (3) a pair of pinacoidal faces 

 also at right angles to the cleavage "and parallel to the longer of 

 the two axes which lie in the cleavage plane. The general form of 

 the cleavage fragments is shown in Plate 4, Fig. 1. Owing to the 

 softness, brittleness, and eminent cleavage, complete crystal forms 

 cannot be detached from the rock for examination, and every attempt 

 in this direction yields at best only oblong hexagonal cleavage 

 plates bounded by these prismatic (dome) and pinacoidal faces. The 

 full form may, however, be largely inferred from the character of 

 the sections of the idiomorphic forms revealed in the microscopic 

 preparations of the rock. These cross sections are remarkably sim- 

 ilar to those which are characteristic of olivine, and at once suggest 

 that iddingsite is a pseudomorph of that mineral. In all references 

 that have been found to this mineral, or to any mineral analogous 

 to it, the writers have invariably regarded it as a result of the alter- 

 ation of olivine.* 



*Most of these references are discussed by Iddings, loc. cit. They are as 

 follows: — 



(1) Dana, Jas. D., "System of Mineralogy," 5th ed., page 465. Description 

 of the mineral thermophyllite, a form of crystallized serpentine- Also ibid, 6th 

 ed., p. 670. The description given of thermophyllite does not accord with that 

 of the characters of iddingsite in several critical particulars. 



Again, ibid, 5th ed., p. 258, we have the following statement: "Alteration of 

 chrysotile often takes place through oxidation of the iron; the mineral becomes 

 brownish or reddish brown and iridescent. It also splits into thin lamina; as 

 the change goes on, sometimes so as to resemble mica. A basalt was once 

 pointed out to the author as a mica slate, although no farther change had 

 taken place than that here mentioned." This statement as to the develop- 

 ment of a lamellar structure is omitted in the recently issued 6th edition. 



