92 REPORTS ON INVESTIGATIONS AND PROJECTS. 



that natural marcasite has formed below this temperature. Pyrite has been 

 made at 6o° by the addition of sulphur to pyrrhotite, also by the action of 

 H 2 S on ferric hydroxide above ioo°. Pyrrhotite is found to be FeS plus a 

 variable quantity (over 40 per cent in artificial preparations) of sulphur 

 which is gradually driven out with increasing temperature until the substance 

 melts as nearly pure FeS at 1180 . Troilite, which is formed in a matrix of 

 pure iron, must therefore correspond to the pure compound FeS, which be- 

 comes the end member of the series of solid solutions. Pyrite dissociates 

 to pyrrhotite, beginning at about 500 , and the vapor pressure reaches one 

 atmosphere at about 68o°. (Review, pages 104, 105.) A definite clue is now 

 established to the conditions of formation of the iron sulphides in nature, 

 and the problem will be pursued further. 



This is the first case in our experience with the minerals in which a solid 

 solution has been formed through the direct action of diffusion. 



PUBLISHED WORK OF THE YEAR. 



Brief reviews of the papers published by the laboratory staff during the 

 year follow : 



(1) The intrusive rocks of Mount Bohemia, Michigan. Fred. Eugene Wright. Mich. 



Geol. Survey, Ann. Rep. 1908, 361. 



A study in rock differentiation. The intrusive mass which outcrops on 

 the south side of Mount Bohemia, Michigan, consists of two distinct rock 

 types: (1) a peripheral shell of oligoclase gabbro, abnormal in mineral com- 

 position; (2) a central mass of oligoclase gabbro aplite. The contact be- 

 tween the oligoclase gabbro and the intruded ophites of the Keweenaw for- 

 mation is usually sharp, while that between the gabbro and the aplite is 

 usually of the transitional type. From granularity relations within the ap- 

 litic mass itself, it is evident, on Lane's theory of the grain of rocks, that at 

 the time of crystallization of the red aplite the oligoclase gabbro was ex- 

 ceedingly hot ; not much cooler, in fact, than the solidifying aplite magma. 

 A consideration of the various possible explanations of the field relation 

 between the gabbro and the aplite favors the theory of separation by frac- 

 tional crystallization combined with convection currents and general upward 

 movement. 



The temperature regions in which the gabbro and the aplite melt in the 

 dry state were found by actual measurement in the Geophysical Laboratory 

 to differ so slightly that no conclusions with respect to a possible eutectic 

 relation between the two could be drawn from the thermal data alone. 



(2) A feldspar from Linosa and the existence of soda anorthite (carnegieite). Henry 



S. Washington and Fred. Eugene Wright. Am. Journ. Sci. (4). 29, 52. I9*0- 



This feldspar, from the volcano Monte Rosso, on the island of Linosa, 

 east of Tunis, is both chemically and physically abnormal; its composition 

 is very closely Na 2 0, 2 CaO, 3 Al 2 O s , 9 Si0 2 , while its optic properties indi- 

 cate a plagioclase of about the composition Ab 3 An 2 . A detailed discussion 

 of the observed phenomena leads to the conclusion that the Linosa feldspar 

 is a plagioclase, containing about 5 per cent of the molecule Na 2 Al 2 O g 

 2 Si0 2 (= Cg) in crystal solution, in which case its formula may be written 



