48 
sition goes to show that they were not originally diorites rather 
than diabases. So far as examined, not a particle of fresh au- 
gite remains ; but an occasional fragment of brownish hornblende 
still exists so far unchanged as to be recognizable. The light 
green viridite is everywhere present, interspersed with magnetite 
grains, particles of epidote and apatite needles. A few of the 
feldspars are still fresh, though here also the viriditie alteration 
has prevailed. My reasons for calling the rocks diabase is that 
the products of decomposition, their form and their arrangement, 
are identical with those of the augitie rocks throughout the en- 
tire area under examination. 
+ 
THIRD SERIES. 
Dike 62.—Very fine grained and compaet; nearly black. 
Under the mieroscope this rock shows a marked deviation from 
the prevailing types of structure as already described. In the 
thin section it shows beautifully fresh and clear porphyritic 
plagioclases, in single lath-shaped forms and in cruciform aggre- 
gates; together with altered olivines and occasional clear but 
corroded augites, imbedded in a dense fine-grained ground-mass 
of small plagioclases, augites and iron oxide. The olivine, though 
changed completely to a chloritic product, is readily recognized 
by its form and irregular fracture-lines. The augites are con- 
fined almost wholly to the ground-mass ; but the section shows 
: few corroded porphyritic forms, as above noted. All are 
fresh and free from enclosures. The iron oxide occurs abun- 
dantly in rod-like forms, crossing one another at nearly right 
angles, produeing grate-like or barred structures. This dia- 
base is the least altered of all that has been examined. 
Dike 66.—Of medium coarseness, and to the naked eye ap- 
parently granular crystalline and fresh. This rock is quite dif- 
ferent in general appearance from any yet described. 
Macroscopically it is a well compacted, quite fresh-appearing 
rock, of , Í š А 
k, of a deep purplish black color, in which hornblende or au- 
