262 TRANSACTIONS OF THE [may 18, 
As stated above, this is a grouping of minerals that might be 
regarded as probably of contact origin. At first sight, the form 
of the mineral deposit seems opposed to this explanation, but 
the objection disappears upon closer examination. For while 
the deposit is an irregular mass, rather than a clearly-defined 
band or zone, it is found to be continuous with such a zone, of 
which it is a local expansion. When the surrounding rocks are 
examined the mineral deposit and the zone of which it is part 
are found to lie between crystalline limestone and a rock of 
totally different character, which, at some points, would be called 
a gneiss, while, at others, it has more the aspect of a massive 
rock, though the gneissoid structure is seldom entirely lacking. 
The occurrence of the minerals upon such a contact, when 
taken in conjunction with their nature, is highly suggestive of 
their formation by contact metamorphism; and that such is 
actually the case is thoroughly demonstrated both by the struc- 
tural relations of tbe rocks in the field and by the character of. 
the gneissoid rock as revealed by the microscope. This rock 
forms rather thin sheets or beds in the limestone, presenting 
much the aspect of interstratified layers; and this, with the 
gneissoid character, at first leads to the idea that the rock is a 
metamorphosed sediment. But when the sheets are traced 
along the strike they are found to cut across and break through 
the limestone in such a way that they must be regarded as in- 
trusive. This is an important point bearing upon other occur- 
rences of a similar nature where the true relations are less 
clearly exhibited. 
Thin sections of the gneissoid rock entirely substantiate the 
conclusion that it is of igneous origin. It consists of horn- 
blende, biotite and plagioclase, with considerable titanite, some 
apatite and occasional grains of garnet. The hornblende some- 
times contains a little colorless pyroxene, evidently all that is 
left of larger amounts of the mineral that have changed to horn- 
blende. This fact, coupled with other cases in the region, sug- 
gests that the rock may have been originally a gabbro; but there 
is no proof that such was the case, and it must be classed as a 
hornblende-mica diorite. The grain and structure vary consid- 
erably, the finer varieties resembling in structure the “ granulitie” 
gneisses, while the coarser have a typical igneous structure. 
In these the plagioclase is slightly older than the ferro-magne- 
sium minerals, so that it shows a rough approximation to lath- 
shaped individuals, while the hornblende and mica have irregular 
outlines, somewhat conditioned by the plagioclase. There is.a 
strong resemblance to the gabbroitie structure figured by Rosen- 
busch. 
