MINERAL CONTENT AND STRUCTURE. 245 
The two kinds of feldspar differ in strength of double refraction, and 
while one is untwinned the other frequently exhibits fine twinning 
lamella after the albite law, whose extinction angles (8 to 10 degrees) 
indicate albite, while the untwinned feldspar seems to be orthoclase. 
The chemical analyses bear out this interpretation. In different indi- 
viduals there is considerable variation in the relative amounts of the two 
feldspars present, but on the whole the amounts are nearly equal. It is 
worthy of note that in individuals in which the albite predominates 
there is a tendency to idiomorphic boundaries against their neighbors 
in which there is no such preponderance. 
The microperthitic structure of the feldspars is sufficiently coarse to 
be readily made out with low powers in all the slides. 
A very slight amount of microcline is present in small anhedra in a 
few of the slides. In no case has it been observed to be microperthitic. 
Undulatory extinction is shown by much of the feldspar, its cause be- 
ing a matter of some uncertainty. There is no sign of cataclastic struct- 
ure in any of the thin-sections, nor any other indication of mechanical 
deformation aside from the undulatory extinction, except that in a few 
individuals, which either consist largely of albite or else have long albite 
spindles, a slight bending of the crystal is apparent. As the large part 
of the feldspar has neither of these structures, by whose aid the bend- 
ing is readily made out, it may be more prevalent than it seems to be 
and be the cause of the undulatory extinction ; but it is difficult to see 
how crystal bending could occur after rock solidification and cooling had 
taken place without the production of cataclastic structure. The fact 
that undulatory extinction is usually a feature of that class of igneous 
rocks whose feldspar is microperthite or anorthoclase would seem to 
justify the query as to whether it might not be due to strains produced 
in cooling, caused by the peculiar make-up of the feldspar. 
Most of the feldspar is crammed with minute, dark inclusions. With 
high powers many of these prove to be specks of hematite, the nature of 
the rest remaining uncertain. Considering the freshness of most of the 
feldspar it is difficult to understand how it could become so filled in 
every part with secondary material, and the inclusions are therefore 
regarded as primary. 
Biotite—This mineral is the most prominent ferromagnesian silicate 
present, occurring in more or less amount in 8 of the dikes, and proba- 
bly originally in 4 others where it has wholly gone to chlorite. In all 
but 4 dikes it is the sole dark silicate present. Though mainly confined 
to the groundmass, phenocrysts have been noted in two of the slides, 
and in others it occurs sparingly included in the porphyritic feldspars. 
It may therefore be fairly said to occur in two generations. It is, when 
fresh, ordinary deep brown biotite, with very small axial angle and the 
