Dikes near Portland, Maine. — Lord. 341 
Under normal conditions the labradorite is, excepting the 
olivine, the first constituent to undergo decomposition. In 
one dike, however (14), it was found remarkably fresh, while 
the augite and olivine were completely decomposed. The 
alteration products are kaolin, calcite. and in one instance, 
chlorite (18). 
Magnetite in the form of well developed octahedra, or as a 
delicate net- work of skeleton crystals (13) is very abundant. 
In some rock specimens this mineral is altered to leucoxene. 
Apatite is of normal prismatic development, and quite 
plentiful in most slides. 
The even granular dikes (7. 10, 17, 24, 26) dififer solely in 
structure from the porphyritic varieties. The mineral devel- 
opment is, wnth the exception 10 and 24. panidiomorphic, like 
the ground mass of the diabase porphyry. In 10 and 24, the 
augite occurs partly as interstitial filling, thus giving the 
specimens a somewhat ophitic structure. 
Enstatite- diabase -porphyry (27, 28) is a coarse grained 
porphyritic rock differing from the olivine-diabase-porphyry 
chiefly in the preponderance of pyroxene over other constitu- 
ents, and by the occurrence of enstatite in place of olivine. 
The augite of this rock is pale green resembling very closely 
the diopside of the andesitic types of diabase-porphyry. (See 
page 340.) 
The crystals of enstatite are of beautiful prismatic devel- 
opment, and have the weak double refraction common to 
this mineral. They vary in length from i to 4 mm, and are 
nearly quadratic in cross-sections, owing to the predomin- 
ance of 010 and 100 over the prism no, which truncates the 
edges of the crj^stals. Cleavage parallel no and 010, and 
parting planes approximately normal to the prism axis are 
common features of this mineral. Along these planes chlor- 
itization is in many cases far advanced (27). 
The chemical composition of this rock (28) is given under 
I, and for comparison that of augite-porphyryte is reproduced 
from professor Kemp's paper.* 
I. II. 
Si02 50.76 5193 
/UO3 1-'. 83 18.13 
*Op. cit., p. 138. 
