Gabbroid Rocks of Mifinesota. — Wifichell. 167 
show that while the crystallization of the pyroxene began some 
time before that of the plagioclase, it was still uncompleted 
when the latter mineral had solidified in large measure. 
The plagioclase, while in general the last of the original 
minerals to solidify, began to crystallize some time before the 
crystallization of the pyroxene had reached its conclusion. 
Thus it is that these two minerals not unfrequently seem to 
show a reversal of the normal order of crystallization. Never- 
theless true ophitic texture is never present, even in limited 
areas, in the normal gabbro. Its appearance marks the pas- 
sage to diabase; all stages of this passage can be found, but 
yet it is nearly always very easy to distinguish the gabbro from 
the diabase. Indeed it is asserted by Bayley* that the granitic 
variety can never be traced into the ophitic variety in the field, 
and that their contact whenever found is very sharp. Lawsonf 
on the other hand, asserts that insensible gradalions from the 
finest ophitic textures to the coarse gabbro texture often oc- 
cur in the thickest intrusive sheets. 
Primary Minerals. The labradoriteX of the nor- 
mal gabbro shows the characteristic twinning, nearly always, 
according to the albite, pericline and Carlsbad laws. The color 
is usually rather dark on account of the numerous inclusions 
of black magnetite needles. The extinction angle in sections 
perpendicular to wg ( =c ) is 33^° to 34° ; in the same sections 
the angle between the albite and pericline twinning is 65°, and 
that between the albite twinning and the trace of the plane 
containing the magnetite needles is 16°. The extinctions in 
sections perpendicular to «p (=a ) is 58° to 585^°, while the 
angle between the albite and pericline twinning in the same 
sections is about jy" or 78°. Such sections cut the magnetite 
needles nearly at right angles, and they appear therefore to be 
mere particles without elongation. The plane in which they 
seem to lie makes an angle of 34° with the albite twinning. The 
acute bisectrix is //g and 2V seems to be somewhat smaller 
than in the Carlton plagioclasyte, but no exact measurements 
* Cf. Bayley loc. cit., p. 453- But Judd (Q. J. G. S., XLI, 1885, 
p. 361, & 1886, p. 40), and others hold the contrary view. 
tA .C. Lawson: Laccolitic sills of the northwest coast of lake Su- 
perior: Geol. Nat. Hist. Surv. Minn. Bull. 8, 1893, p. 30. 
JA detailed study of the labradorite of the plagioclasyte will fol- 
low the description of that rock. See chapter IV. 
