LITHOLOGY. 173 
They show all the structures and the outlines of the original crystals tea 
characterize diabase, but every crystal is altered, and converted into 
an aggregate. A determination of the silica in the specimen from the 
Lincoln flume gave 40.04 per cent. Allowing for the carbonic acid and 
water present, this indicates no material increase in the amount of silica 
and the consequent presence of most of its original constituents, for the 
undecomposed rock at the Lincoln flume, the analysis of which is given 
on p. 153, is just like the one from Bemis brook. This decomposition pro- 
duct, however, contains a large proportion of carbonates of lime, iron, 
and magnesia. The size of the flume shows how much of this dyke has 
been removed; and when this decomposition product is broken down 
and carried away, these materials will be separated—the lime and mag- 
nesia will be carried to one place, the iron to another, and the siliceous 
residue to another; and this rock will be broken up into portions, which 
will be in part more basic, and in part less basic than the original rock.* 
The details of these processes of decomposition are as various as are 
the minerals and the circumstances that act upon them; but the gen- 
eral result is always the same, and hence this case may be taken as 
typical of the processes which have operated on the original basic crust 
of the earth to break it up and assort its materials into more and less 
basic portions. 
ACIDIC UNSTRATIFIED ROCKS. 
In approaching this great family of rocks, which with the crystalline 
schists forms our mountains and hills, some of the considerations 
drawn from the study of the basic eruptive rocks are of value. It was 
Stated that of such basic material the original crust of the earth was 
probably formed, and that by the ordinary processes of decay the bases 
of such rocks are in part removed, are accumulated in the sea and in 
beds of limestone and iron ore, while a siliceous residue is left behind, 
or is washed away and accumulated in sedimentary beds. Beneath these 
secondary products the original crust of the earth is now so deeply bur- 
ied that basic eruptive rocks are the only possible representatives of it. 
In some regions the limestone derivatives cover the whole surface; but 
ee ears 5 “ 
See, also, in this connection, the analysis of anorthite, on Page 91, and the remarks in connecti: ith 
ton with it, 
