MINERALOGY AND PETROLOGY. 967 



The globular state of incipient minrrals.] 



most common in the muscovadyte, or noryte stage of the gabbro. In this rock, 

 in proportion as one or the other mineral is able to develop more rapidly, it grows 

 beyond the globular state and poikilitically surrounds the globular incipient grains 

 of all the others. This is true of biotite (Nos. 1092, 1777, 638G, 740G), of quartz 

 (Nos. 1039, 1339, 1340, 8M), of magnetite (Nos. 677, 1340, 1343, 2201), of olivine 

 (Nos. 916, 1367, 1343, 505H), of labradorite (Nos. 677, 767, 1345, 1366, 2203, 638G), of cor- 

 dierite (No. 1092), of augite (or diopside) (Nos. 677, 767, 1092, 1336, 1341, 1089, 2197), 

 of diallage (Nos. 1340, 847G, 857G), of hypersthene (Nos. 960, 1343, 1364, 2203), and of 

 amphibole (Nos. 767, 1345, 2197, 2209, 638G). These included globular grains cannot 

 be considered in all cases as the infantile starting points of the minerals they repre- 

 sent, but in many cases they are grown beyond the infantile dimensions, though 

 they have retained their infantile roundness. It can be seen also that in some 

 instances the formation of a large crystal which poikilitically embraces several 

 others, is made to embrace many minute globular forms of its own kind (Nos. 1092, 

 1343, 2198, 2209). These latter are very evident when they do not stand in the same 

 orientation as the enclosing crystal (Nos. 2198, 2209), but when they have placed 

 themselves in the same orientation they appear to be lost in the general growth. In 

 case of porphyritic crystals developing from a magma, such increments to the growing 

 crystal would be free to arrange themselves in accord with the polarity of the main 

 crystal. In a plastic or semi-plastic mass, or in a solid undergoing a forced, slow 

 transformation, the various points at which crystallization might simultaneously 

 spring up would be so obstructed by their surroundings that they could not all 

 assume crystalline agreement with each other. 



This feature has been observed also in the rhombs of siderite that appear in the 

 jaspilyte of the Lower Keewatin (Nos. 388, 903, 907, 1565) and in that which accom- 

 panies the taconyte of the Taconic (No. 1307). 



It appears justifiable, therefore, to regard this early globular state of the con- 

 stituent minerals of a rock as an evidence of metamorphic origin of the rock. Even 

 if this character be seen in but one of the minerals of a given rock, it is evidence 

 of transformation, and all the minerals of such a rock could be looked upon as 

 probably having been developed from an earlier state of the rock. 



Owing to the irregular manner of growth which minerals under metamorphic 

 forces are compelled to follow, such grains might be mistaken for " crushed " crystals, 

 and the roundish small parts which have not been able to unite with the larger 

 crystals in uniform orientation might be, and have been in some cases, taken for the 

 debris formed by such crushing. They have also been ascribed to a supposed 

 peripheral action of a flowing igneous rock, the cause being presumed to be the 

 motion of the mass in separating, if not actually breaking, the initial crystals of the 

 different minerals. 



