63 



mérous vicinal faces; but they are, in ail instances, linnited to 

 a shorter extent of the zone. 



A frequent consequence of the presence of numerous vicinal 

 faces is that the faces of the zone become striated; in the 

 present type, howewer, this striation in most instances is not 

 found. 



It is often very difficult to decide whether in a certain 

 instance we have before us a vicinal face or not. The fact is 

 that we can get no fixed starting point with regard to the 

 measuring, as no single face is quite free of vicinal faces. 

 When for the same angle in different instances, considerably 

 varying values are found, even if all the single reflexes be 

 absolutely perfect, we may come to the conclusion that the 

 deviations are due to the presence of vicinal faces ; but whether 

 they replace all the measured faces or only a few of them, and, 

 if such is the case, then which of them, cannot be decided 

 with certainty. When for several of the angles of a zone values 

 are found, agreeing tolerably well with the average value, and 

 when a single face then gives especially deviating angles, the 

 predominant probability will of course be that this face has 

 been replaced by a vicinal face ; such angles deviating up to 2° 

 from the calculated value, have not been included in the table 

 of angles given above. 



As to the separate zones, the zone [010,001] seems upon 

 the whole to be the best developed one , and therefore one 

 fundamental angle has been chosen in this zone. It is very 

 rarely the case that the reflexes given by the faces /'{04l} 

 and e(02l} consist of two or more sharply separated signals, 

 which is often the case in the other zones. The values of the 

 angles found by the separate measurings, may deviate rather 

 considerably from each other; but the inaccuracy is not, how- 

 ever, nearly so large as in any of the other zones, and some 

 part of it may perhaps also be due to casual irregularities in 

 the inner structure of the crystals, and if so, it will be elimin- 



