130 



WARREN. 



which is in the neighborhood of 70° with tlie basal cleavage. As a 

 whole their direction is more uniform than in basal sections although 

 more or less variation is to be seen, particularly in samples in which 

 the albite bands are less numerous. The bands in general are more 

 continuous and are probably less prone to branch and coalesce than 

 in basal sections. 



The contacts with the niicrocline are seldom smooth; generally 

 they are irregular and sometimes finely serrated. Minute projec- 

 tions extend out from the band into the microcline following the 

 principle cleavage directions, most commonly that of the base. These 

 may in rare instances extend out for some distance or even to the next 

 parallel band. Beside the main lamellae there is without exception 

 a series of exceedingly minute lamellae which run parallel to the g- n- 

 eral direction of intergrowth. In some sections there is perhaps a 

 gradation of these into the larger ones but generally they appear to be 

 of a different order of magnitude. Much less commonly minute 

 lamiellae or flakes of albite may be seen following still other directions 

 than those already mentioned, but these are unimportant. 



In those perthites which are poorer in albite the lamellae appear in 

 general to be more irregular in direction, distribution, width, and in 

 form, than in those richer in albite. For the perthite from a given 

 locality, however, the general features of the intergrowth taken as a 

 whole are uniform and are quite characteristic. 



In all the feldspars here studied the potassic feldspar is a microcline. 

 In fact the WTiter has never yet seen a perthite which was not a micro- 

 cline perthite, and doiibts very much if there is such a thing as a 

 pegmatitic perthite which contains orthoclase. Usually the charac- 

 teristic polysynthetic twinning is present. In many cases it may be 

 very faintly developed or missing altogether. This last fact may 

 account for the statement, so frequently made, that the potassic 

 member is orthoclase. It is worth noting in this connection that the 

 potassic feldspar in many granitic microperthites shows the same 

 characteristics, and also sometimes shows the albite twinning without 

 the pericline. The optical properties are always those gWen for 

 microcline so far as the writer's observations have extended. 



A somewhat general peculiarity of these perthites is that cleavage 

 surfaces are rarely perfectly continuous. Small variations of level 

 may be noted from place to place. This slight deorientation has 

 doubtless been caused by crushing stresses whose effect is also some- 

 times recorded in the slight deorientation of parts of the microcline 

 (less commonly in the albite) in certain feldspars as seen under the 



