136 The American Geologist. March, isoo 
in which high powers show to be made up of dusty material 
segregated there. (83) The olivine is in relatively large idio- 
morphic crystals, and is in size the largest of the rock com- 
ponents, running from 1-4 m m. A number of measurements 
indicated 2Po6 and ooPoo as the most common faces in sec- 
tions. As a matter of course, it always shows strong begin- 
nings of alteration to serpentine, and more often than not, the 
serpentine alone remains. This alteration product affords at 
times the interference crosses of radial aggregates (No. 15). 
The last stage leaves a core of calcite surrounded by a radiat- 
ing rim of serpentine. The olivine preceded all the others ex- 
cept the magnetite in formation. The magnetite is in grains, 
often showing octahedral outlines, and at times clearly titan- 
iferous. The most interesting of all is the hornblende. As 
noted above, it appears in not a few of the dikes. It is some- 
times idiomorphic, exhibiting ooP and ooPoo" with all the char- 
acteristic pleochroism usual to the brown basaltic variety. 
When this is present in abundance the dike approximates the 
typical camptonites very closely. Dike 22 could hardly be dis- 
tinguished in the slides from Hawes' olivine-diabase from 
Campton Falls. Again the hornblende surrounds the augite, 
or is such a close part of it that a paramorphic origin from the 
augite is an irresistible conclusion. The widening applica- 
tion of microscopic study has shown this to be a not uncom- 
mon phenomenon, and in American dikes has it been noted by 
those mentioned below.' 
The porphyritic types of structure occur under two widely 
differing relations. The exceedingly narrow dikes, either 
those that taper out to an edge in the country rock, or those that 
have oozed into cracks in larger and partly cooled dikes, al- 
ways show a porphyritic structure, which is easily attributable 
to their quick cooling. But strangely enough, the broadest 
dike of all (92) and others of no inconsiderable width, are 
likewise porphyritic. The greater number must be classed as 
belonging to the melaphyre type (Nos. 10, 12, 32, 50, 66, 79, 84, 
iG. W. Hawes, N. H. Surv. Vol. Ill, pp. 57-206, PI. VII, fig. 1. 
W. H. Hobbs, Bull. Mus. Comp. Zool., Vol. XV. No. 1. p. 10, Som- 
erville, Mass. 
A. C. Lawson, Proc. Can. Inst. Apr, 1888. Vol. 23, p. 173. Eainy 
Lake Region. 
E. Haworth, Amer. Geologist, May and June, 1888. Missouri. 
In larger rock bodies it is a well known association. See references 
in Hobbs' paper. 
