Smyth — Crystalline Bocks of St. Lawrence County. 



491 



them. This happens, as shown above, in the case of limestone and gneissic 

 layers, but in the present case the rocks involved are too much alike to permit 

 the acceptance of such an explanation. This objection holds good against the 

 first supposition — that of interbedded layers. 



The possibility of the black bauds being segregations in an igneous rock 

 is, for the typical cases, excluded by their form, although it may be applicable 

 to some occurrences. There remains the supposition that the black bands are 

 fragments of an older gneiss, included in a gneiss of igneous origin. This 

 explanation is the only one that appears to be in harmony with the facts, and 

 free from serious objection. According to it, the bands owe their shape to 

 their breaking from the parent mass, as they would, in the direction of 

 least resistance. Their occurrence in groups is just what would naturally 

 follow from such an origin, and the same is true of the irregular scattering of 

 these groups. The parallel arrangement of the neighboring bands doubtless 

 results from currents in the molten magma, which would tend to produce such 

 a result. It is probable that the breaking into blocks resulted, in part, 

 from strains applied after the magma was in a pasty and partially crystallized 

 state. The blocks were more or less widely separated, and the intervening 

 space was tilled by the magma, which flowed around the blocks without 

 destroying their angular contour, and, at the same time, often produced an 

 obscure flow-structure in the gneiss, parallel to the sides of the inclusions. 

 The tine fissures and cracks were tilled with the more acid portions of the 

 magma, which were hist to crystallize, and were strained into these cracks, 

 producing the coarser pegmatitic veins. These minor details, are not, of 

 course, in the least essential to the explanation, although suggested by the 

 phenomena observed in the held. The supposition as a, whole accounts for 

 the facts stated, and has, as yet, no strong evidence against it. 



The irregular black masses occur in much the same way as do the bands, 

 differing only in form and size. They are sometimes nearly circular, or 

 elliptical, but more often extremely irregular, with an outline marked by pro- 

 jections, and dee}) embayments. They often attain dimensions much greater 

 than those of the bands. The foliation is often pronounced, quite commonly 

 more so than in the surrounding gneiss. The two foliations, that of the black- 

 masses and of the gneiss, range from parallel with, to perpendicular to 

 each other. Where the included mass is decidedly elongated in one direc- 

 tion, this is usually parallel to the foliation of the surrounding gneiss. The 

 foliation of the gneiss sometimes follows the sides of the black masses, as 

 though it had flowed around them, and not infrequently, narrow, irregular 



