Secondary Occurrences of Magnetite. — Kimball. 16 
ognize in secondary anfractiiosities from contraction and ex- 
pansion of the mass incidental to hydro-chemical mutations, 
and again in such as subsist between divisional surfaces ofall 
other kinds, especially when acted upon by corrosive solutions 
in secular transmission. F'rom another point of view, sir«-h 
loci of deposition whether macroscopically or microscopically 
considered, are virtually determined by mechanical condi- 
tions of least resistance as effected by both chemical and phys- 
ical forces operating alternately or together and developed one 
from the other. Not least among these correlative forces to be 
recognized are molecular or concretionary attraction of the 
mass for homologous matter in passing solutions, or even in 
suspension; or, a"? I have elsewhere used the term, the extra- 
mnlecular tendency of ochreous material to form concretions 
or aggregations.* 
Union of magnetite molecules is doubtless promoted also b}^ 
polarity. Incidentally to common secular weathering of bas- 
ic rocks, the chemical forces brought into play are easily per- 
ceived to be mainly through simple alternating and regener- 
citive reactions in which atmospheric oxygen and carbonic acid 
take the lead. In a valuable contribution to the studjMjf this 
subject, Mr. J. E. Spurr has partially traced with much 
minuteness the cyclus of phj^sical and chemical operations 
concerned in the isolation and fixation of ferriferous oxides 
from basic rocks. j- 
Secondary ore deposits of the types above referred to were 
instanced by me in tiie year 1884, as illustrated by important 
occurrences on the south shore of the island of Cuba, between 
the buys of Santiago and Guantx'inamo.+ Numerous geolo- 
gists (Pumpelly, Van Hise, the Messrs. Winchell, and others) 
*Am. Jour. Sei., XLir, 240. 
IGeol. Surv. Minn., Bull, x. Chapters vi and ix. Vid. Senft. Min.. 
1875, p. 502. 
Changes, however, from magnetite to hematite and siderite, and a- 
gain from hematite to siderite. as conceived and syml^olically expressed 
by Mr. Spurr, are impossible reactions without intermediate reduction. 
The common transformations to i)e considered with relation to change 
of volume are namely, of combined ferroufe oxide to hydrous ferrous 
carbonate and ferric liydrate, and of anhydrous ferrous carbonate to 
ferric hydrate and anhydrous ferric and magnetic oxides. Spurr. 
op. cit. 162. 
JAm. Jour. Sci., xxviii, 41(5; Trans. Amer. Geol., Min., Eng., xiii. 
