84 */. M. Clements — Study of Contact Metamorphism. 



gate is presumed to be of clastic origin, and to be chiefly feld- 

 spathic dust, darkened by carbonaceous and ferruginous specks. 

 Careful search was made for flakes of chlorite and biotite, but 

 no traces of either mineral could be found. Likewise, feldspar 

 is absent in grains or crystals of a size which could be recog- 

 nized. The banding of the slates is occasioned by the varying 

 quantities of these minerals, although each band contains some 

 individuals of all the kinds enumerated. All the crystals lie 

 with their long axes parallel, causing the cleavage of the slate. 

 It is clear that the slate is already in what we may perhaps call 

 a semicrystalline state, the crystals of white mica, actinolite 

 and rutile evidently being new developments in the sediments. 



Origin and composition of the clay-slates. — The origin of 

 the clay-slates of the Mansfield formation is probably to be 

 looked for in the disintegration and decay of the Archaean 

 granite and the subsequent compression of the resulting clay ; 

 for between the Archaean granites and the Mansfield slates no 

 other rock masses are known to have existed in this region 

 from which the clay could have been derived. 



An examination of series of analyses of granites shows that 

 while the relative percentages of soda and potash vary con- 

 siderably, on the whole the potash is the higher of the two. 



As a consequence of the easier solubility of the soda, this 

 relation between the alkalies, soda and potash, is maintained, 

 and is often very striking in clay-slates. An average of thirty- 

 one analyses of clay-slates taken from various sources shows 

 two and one-half times as much potash as soda. In the case of 

 the Mansfield slate, it will be seen that the difference is much 

 greater, there being present ten times as much potash as soda. 



Analysis of Mansfield Slate. 

 (By Mr. Geo. Steiger, U. S. G-. S.) 



Si0 2 ... 60-28 



Ti0 2 -69 



A1 2 3 _ 22-61 



Fe 2 3 2-53 



FeO ___ _ -45 



MnO trace 



CaO _ ._ -13 



BaO -04 



MgO 1-35 



K 2 ._ 5-73 



Na 2 _ -54 



H 2 at 100— '60 



H 2 Oatl00+ 3-62 



P.O. -03 



C0 2 none 



C -97 



Total... 99-57 



