3o6 The American Geologist. November, mn 
normal biotite. Gariitt and sillimanite have not been found 
in the cordierite noryte from Minnesota. Finally the hyper- 
sthene of the gabbros and norytes of Pallet is changed by 
amphibolization to the monoclinic type cummingtonite, while 
the bronzite of the Minnesota rock by the same process 
changes to the rhombic type anthophylllite. This change is 
very similar to that described by M. A. Lacroix'in the noryte 
of Arvien, but differs in that the original pyroxene in the latter 
case is negative hypersthene instead of positive bronzite. 
It is well known that cordierite often contains pleochroic 
halos about minute included crystals of zircon. Such halos 
are found in the cordierite of the noryte of Pallet, described by 
Prof. Lacroix. In the Minnesota type, zircon causes halos 
in the anomite, but it has not yet been found enclosed by cor- 
dierite, which is wholly devoid of halos. 
Oric;in. It is considered here that the cordierite noryte 
is an endomorphic form of the gabbro of the region. This i? 
demonstrated by the field relations which have been described, 
but it is not less clearly proved by the mineralogical an:l 
chemical composition. The excess of alumina which gave 
birth to cordierite and biotite is evidence of the contact origin, 
since such an excess is practically unknown in basic rocks 
except as produced by absorption of aluminous elements. The 
cordierite, indeed, is a mineral, as previously stated, whose 
occurrence in. basic rocks is wholly exceptional. 
The presence of quartz indicates an acidity in the magma 
which could hardly be produced at the border of the mass by 
differentiation. 
Finaly staurolite, epidote and spinel are minerals especially 
abundant in regions of contact, and in the met amorphic series. 
The probable presence of graphite, included within the 
staurolite, adds another mineral to this list of contact products. 
*A. Lacroix: Mineralogie de la France, I. p. 558. 
iTo be conii)uicd.\ 
