60 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



in the g-reater quantity. As a rule, in all of the igneous rocks (and to 

 the igneous rocks all clay slates owe their ultimate origin), except in the 

 nonfeldspathic ultrabasic ones, the reverse condition exists, namely, the 

 magnesia subordinate in quantity to the lime. The difference in amount 

 of soda and potassa is very striking and should Ije noticed, in view of cer- 

 tain points to which attention will be called in subsequent pages. The 

 percentage of alumina is higher than is usual in the clay slates. It will be 

 noticed that considerable water is present, but in consideration of the char- 

 acter of the rock this is to be expected. If anything, the value is rather 

 lower than would be expected, indicating a possible loss of water due to the 

 rock having already undergone some dynamic action. The carbon present 

 is considered as offering trustworthy evidence of the presence of organic life 

 at the time of the deposit of the slates, though no more satisfactory evidence 

 of the existence of life has been found. 



COMPARISON OF ANALYSIS OF MANSFIELD CLAY SLATE WITH ANALYSES OF CLAYS. 



During the last few years there have appeared in the Geological 

 Magazine, from the pen of Mr. W. Maynard Hutchings, some very elaborate 

 and suggestive articles upon the composition of clays, shales, and slates, 

 and from one of these ^ I have taken two analyses of Carboniferous clays 

 for comparison with the Mansfield claj^ slate. These two analyses, Nos. 2 

 and 3, p. 59, are from the very fine grained clays, in which the quartz was 

 not distinguishable with the microscope, and are the analyses showing the 

 highest and lowest percentages of silica. Mr. Hutchings says of his 

 analyses that the samples were dried at 220° F., and that the titanic oxide 

 was not determined but is contained in the silica and alumina. Concerning 

 the clays, he writes: 



From these analyses it will be seeo that these clays would be capable, chemically 

 considered, of transformation into very typical "clay-slates." Mineralogically they 

 are clay-slates, having already undergone all, or nearly all, the mineral changes 

 requisite to constitute the normal (unaltered) slates. Nothing more is needed but 

 physical chauges, such as compacting, arrangement of mica in a plane, increase of 

 sise of mica, etc.^ 



The great similarity of these days with the Mansfield day slate is very 

 evident. The only material difference which exists between them is in the 



'Notes on the compoaitiou of clays, slates, etc., autl ou some points in their contact metamor- 

 phism, by W. Maynard Hutchings : Geol. Mag., Vol. 1, 1894, p. 38. 

 ^Loc. cit., p. 38. 



