148 . The American Geologist. Marcii, i898 
thin sections show that the cleavage and fracture planes of 
the primary minerals as well as the interstices between them 
are more or less completely filled with these alteration pro- 
ducts. No titanic acid is discovered in the rock. The small 
plagioclases are the only original constituents whose outline 
and original character are even partially preserved where mag- 
netite segregation has been most energetic. In the fresher 
portions of the diabase the plagioclases are clear and com- 
paratively unaltered; in the more advanced stages of altera- 
tion, quartz and epidote pseudomorphs preserve the original 
crystal form in considerable perfection. The staining due to 
ferric oxide is common to all phases of these rocks. Horn- 
blende is not abundant. One of its most interesting develop- 
ments is a fibrovis vein-hlling which in turn is being replaced 
by secondary quartz. There occurs sparingly distributed in 
certain altered phases of the diabase, and quite abundantly 
developed in the accompanying fragmental beds, a fibrous 
secondary mineral identified as actinolite. It occurs most 
commonly piercing the quartz grains which are abundant in 
these altered phases. 
Summary. As a summary of the observations made on 
the alteration tendencies of these rocks, the following state- 
ments can be made: 
First. — There is an alteration towards quartz and epidote 
leading to a most indestructible rock. The upper zone of each 
flow is more subject to this alteration than any other portion, 
although the inclination to this change is not confined to any 
particular part of the flow. The rock always assumes a yel- 
lowish green color in this stage. 
Second. — There is an alteration toward a highl)' ferrugi- 
nous rock in which the usual secondary minerals are pres- 
ent, but in which secondary hematite and magnetite are ac- 
cumulated in great abundance along all boundaries and frac- 
tures of the original minerals. The color of the rock be- 
comes then a dense black. 
Third. — There is an alteration toward a kaolinized earthy 
mass in which the only minerals determinable are quartz and 
kaolin and an iron oxide with more rarely copper carbonate 
stains. This phase is almost wholly confined to the basal con- 
glomerates in which the conditions for disintegration and the 
