LITHOLOGY. 1/3 



They show all the structures and the outlines of the original crystals that 

 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 shovv's 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 hmestone derivatives cover the whole surface; but 



* See, also, in this connection, the analysis of anorthite, on page gi, and the remarks in connection with it. 



