PETROGEAPHICAL CHARACTER OF SIAMO SLATE. 319 



In the least mashed pliases ot" the graywackes there appears to Ije no 

 arrangement of the secondary leaflets of chlorite, muscovite, and sericite in 

 any definite direction. Where the dynamic action was somewhat greater 

 there is a suggestion of the arrangement of the leaflets of these minerals 

 in a parallel direction; also the original grains of quartz and feldspar are 

 mashed or somewhat rotated, so as to have a similar arrangement. Further, 

 finely crystalline secondary quartz begins to appear prominently in the 

 background, and chlorite, which was predominant in the less mashed phases, 

 becomes less prominent, being replaced by biotite and sericite or muscovite. 

 Where the dynamic action was somewhat more severe the slides show 

 distinct evidence of minor fault-slipping along two sets of diagonal planes, 

 the somewhat irregular, connecting, and mesh-like slip-planes being marked 

 by continuous bands of chlorite and mica, mingled with oxide of iron. 



The chloritic and micaceous slates differ from the graywackes only in 

 that the distinctly recognizable fragmental quartz and feldspar are much 

 less abundant and the matrix much more abundant. As the quartz and 

 feldspar grains become of very small size they are less rounded, apparently 

 being below the limit of magnitude affected by water action. Out of the 

 feldspar there formed chlorite, biotite, sericite, muscovite, and quartz, exactly 

 as in the graywackes, and the same minerals also developed in the matrix. 

 On account of the more plastic character of these rocks the evidence of 

 interior movement is much greater than in the graywackes, the inter- 

 secting slip-planes being more numerous and approximately parallel, like a 

 drawn-out net. 



In a more advanced stage of alteration the slip-planes increase in 

 number and are more nearly parallel, until there are several or many 

 in the breadth of a single millimeter, and here we have typical fissility. 

 The chlorite and mica developed or were arranged parallel to the fissil- 

 ity. The slip-cleavage very often corresponds with bedding. It appears as 

 though different layers had been pushed forward over one another, some- 

 what as are particles of dough under the roller, the elongation being g'reater 

 in the direction of the movement of the roller and less at right angles to 

 this in the plane of movement. Sometimes there are present large flakes 



