FISSURES FILLED FROM BELOW. 433 



the planes of stratification in micaceous sandstone, and would stratify the 

 dike transversely. It has been shown, however, that the scales of mica in 

 the dikes do not lie horizontal but stand on edge vertically, parallel to the 

 sides of the dikes, and that the banding which is in several dikes very dis- 

 tinct has the same position. It is evident from these facts that the fissures 

 were not filled from above by ordinary sedimentary processes, but that the 

 sand was forced into them. 



The arrangement of the scales of mica parallel to the sides of the dike is 

 the one of least resistance, and is a natural consequence of the motion of the 

 sand as a body in the fissure. It appears to be analogous to the fluidal 

 arrangement of crystals in eruptive rocks. So far as the position of the 

 mica and the banding are concerned, the motion may have been in any direc- 

 tion within the plane of the dike. 



That the sand has actually been forced into the fissures is shown by the 

 effects produced upon the form of the scales of mica. Attention has already 

 been called to the fact that many scales are crushed edgewise, as represented 

 in figure 5. In this case the direction of motion is not evident, whether 

 upwards or downwards in the dike. For the purpose of discovering evidence 

 concerning the direction of motion in the sand, three thin sections (one hori- 

 zontal, another vertical aud transverse, and a third vertical and parallel to 

 the dike) each were prepared of a number of dikes, and a study of them has 

 thrown considerable light upon the subject. It is easy to understand that 

 owing to the friction upon the walls of the fissure the sand in the middle 

 would move more rapidly than that upon the sides, and in this way a 

 shearing strain would be set up in the grains by their mutual attrition. If 

 this strain distorted the grains it is evident that the form of the distortion, 

 considering also its position in the dike, would indicate the direction of 

 flowing in the sand. In one of the vertical transverse sections the phe- 

 nomenon represented in figure 6 was observed, and conclusively demonstrates 

 that the motion of the sand in filling the fissures and forming the dikes was 

 from below upwards. 



It must not be forgotten, however, that the vertical position of the mica 

 scales, as in many metamorphic rocks, and the banding also, could probably 

 be produced by movement in the mass as a result of lateral compression after 

 the fissures were filled with loose sand. But there is no need of appealing to 

 lateral compression, for the movements at the time the fissures were filled 

 will explain all the appearances. 



A number of dikes fail to reach the surface, and others are offset in such 

 a manner that it would seem impossible to fill the fissures from above. These 

 facts strongly support those already adduced, and render it certain that the 

 sand was forced up from below to fill the fissures. 



