2o6 Lawson on diabase dykes of the 'Rainy lake region. 
morphic porphyritic crystals in the fine grained parts near the 
contact and its absence in the coarser central parts; the diminu- 
tion in size of the porphyritic crystals near the contact in co-ex- 
tension with the increasing fineness of the ground mass; and 
finally the "jDoly somatic" structure common to the augite 
throughout the dyke. 
Three quarters of a mile from the exposure where the speci- 
mens whose characters have just been given were collected, 
there occurs, on the opposite side of the bay in the line of the 
strike of the dyke, another exposure of the same dyke. On the 
islands of the bay which lie intermediate between these two lo- 
calities the outcro2) of the dyke is observable, so that there is 
no doubt of their both being exposures of the same dyke. The 
rock here was not studied in so great detail as at the last expos- 
ure. The specimens taken were of the same grade of coarseness 
as those taken at 30 feet from the contact on the north side of 
the bay. The feldspars are more decomposed and the twinning 
lamelhe often obscure, and the small quantity of quartz wdiich 
is associated with them appears to be of secondary origin ;. 
whereas the origin of the quartz noted in the same dyke on the 
north side of the bay seemed much more problematic. In the 
latter case the common micropegmatitic character of the quartz 
and the bccurrence in it of needles of apatite, which in no way 
differ from those in the feldspar, together with the not infre- 
quent occurrence of one individual of apatite partially included 
in quartz and partially in adjacent feldspar, would argue for the 
primary character of the quartz. The augite in the dyke on 
the south side of the bav resembles that already described occur- 
ring both in simple individuals and in polysomatic masses. It 
is largely altered to uralite. Titanic iron with its alteration 
product leucoxene shows characteristic barred structure of the 
cleavage traces parallel to the planes of the rhombohedron. The 
leucoxene is frequently accompanied by a margin more or less 
extensive, of secondary brown mica. Apatite is present in com- 
parative abundance. Chlorite occurs in vaguely defined masses 
and the garnets which, as before, are present, are associated 
with it. 
On the south-east shore of Pipestone lake about a mile west 
of Stone-dam Portage occurs another of these dykes cutting 
