24:6 Prof. Bonney : Penological Notes on the Euphotide 



and diallage (fig. 3) . In parts of this, especially in the pyroxenic 

 constituent, a slight " streakiness " is exhibited, in others an 



Fig. 3.— Bough sketch of block showing a coarse gabbro streaking- smarag- 

 dite-euphotide. The upper band is about 5 inches thick. 



ophitic structure can be traced. In this the eC crystals v of 

 saussurite are sometimes 2'5 or 3 inches long : the patches of 

 the pyroxenic mineral being even as much as 4 inches. The one 

 form shades off into the other by imperceptible changes, the 

 crystals of saussurite tending to assume a parallel position and 

 to become, as it were, distorted. This coarse and slightly 

 " orientated " band on the right-hand side of the block changes 

 with great rapidity into a rather fine-grained, less pyroxenic 

 variety of euphotide ; the diameter of the pyroxenic constituents 

 being about *25 inch at most, that of the saussurite a very 

 little more. The structure of this rock is granular, and only 

 here and there we detect a slight streakiness. But on the 

 left-hand side of the block, as shown aboTe, we find a band, 

 about half a yard long and three inches broad, of a gabbro as 

 coarse as that of the upper zone, and exhibiting in its crystals 

 a similar, perhaps a shade more marked, tendency to orienta- 

 tion. The one is separated from the other by a band like the 

 main mass of the block, though a little coarser and more 

 pyroxenic, but at last the lower one " streaks " upwards to the 

 upper. There is no sign that the one variety either has been 

 made by a local crushing of the other, or is intrusive into it ; 

 but we observe a rapid transition from the one to the other, 

 as in a rock which originally consisted of two magmas slightly 

 different in composition. 



Boulders are common in which there is a streaking or vein- 

 ing, without absolutely sharp boundaries, of finer varieties by 

 coarser, or of those which are more felspathic by a rock richer 

 in a pyroxenic constituent — augite, hornblende, or smaragdite. 



Boulders also may be found occasionally in which bands of 

 compact greenstone cut across the streaky structure in a 

 euphotide, so as to prove that, whatever might be its cause, 

 this structure existed at the time of the intrusion of the 

 greenstone. 



