ORIGIN OF AMPHIBOLITES OF LAURENTIAN AREA aly) 
as that of the adjacent Methuen bathylith, the analysis of which is 
given below: 
SHO ees Oe a ge ene heise deh Aen Cr om ener Aa res ee 1.66 
“LESK ©) 325 SS ia ma a I ee Q}SStb a oT Had © VERT atest Hie ey cree 0.45 
Jel OSE cic, eS nA, Ma a uO ara OSG en O) ean else eee y was Mantes nse 
IetSK ODE ae i ete ei Bea Oe Om an OrsG Suan Nici ONrn cipe way cong retard. deriva 5 -O1 
19C(O) Se ose Re UO Ree TPG we EL a) yee ott ceric Ae eua re Nets 0.45 
MnO 0.04 99-89 
The changes are the result of the transfusion into the limestone 
of certain constituents which are present in the granite magma. A 
remarkable fact in connection with the alteration, is that the granite, 
which is an acid variety of the rock containing a very small amount 
of biotite as its only bi-silicate, where the limestone was bathed by it 
or actually immersed in it as in the case of the included fragments, 
has notwithstanding this fact transfused into the limestone not only 
silica, alumina, and alkalis, as might be expected, but also large 
amounts of magnesia and iron. ‘The limestone evidently fixed certain 
constituents of the granite magma in relatively greater abundance 
than others, exerting a species of selective action. Many cases have 
been described in which a granite magma has passed by differentia- 
tion into a gabbro, but here the granite retains its normal character 
and at the same time changes the limestone into a rock having the 
composition of a gabbro. 
That similar changes are brought about by the action of acid 
magmas upon limestones elsewhere is shown by two occurrences 
described by Kemp! and one by Lindgren. The first is from San 
José, in the State of Tamaulipas, Mexico, the second from Morenci, 
Arizona, and the third from White Knob, Idaho. In all cases highly 
acid intrusive rocks, quartz-porphyries or quartz-diorite-porphyries, 
very low in iron, penetrate limestones which are so pure that they 
_can yield little or no garnet of themselves. In each case the intrusives 
tJ. F. Kemp, ‘“‘Ore Deposits at the Contacts of Intrusive Rocks and Limestones 
and Their Significance as Regards the General Formation of Veins,’ Economic 
Geology, Vol. II, No. 1, 1907, p. 1, and Trans. Am. Inst. Mining Eng., XXXVI, p. 192. 
W. Lindgren, U. S. Geological Survey, Professional Paper No. 43, p. 134. See also 
O. E. Le Roy, ‘‘The Marble Bay Copper District,” Jour. Can. Min. Inst., 1907, 
p- 248. 
