132 .1. s. DILLEB — SANDSTONE DIKES 



w.l] In- thai the joint- were formed at about the same time as the dikes s 

 both beiog results of the -aim' general cause; but it is clear that the joints 

 were formed before the dikes, and that the dikes may be regarded simply as 

 large join! fissures tilled with Band. There is a complete gradation in the 

 size of the dikes, from a mere film in a joint, as shown in plate I, figure 3, 

 up t" s feel in thickness. 



\fethod of filling the Fissures. — Fissures in rocks may be filled with mat- 

 ter brought into them in the gaseous, liquid, or -olid state. When tilled by 

 the crystallization of minerals from a gaseous or liquid condition, either 

 that of solution or fusion, the rock produced in the fissure must he more or 

 less crystalline and easily distinguished from one formed by filling a fissure 

 with solid particles or fragments of minerals. In the first case the mineral 

 or mineral- crystallize in place, and if there is no interference in the process 

 crystals will develop more or less perfectly, a- in the rocks of many igneous 

 dikes. Bach particle is bounded either by crystal planes or less regular out- 

 line- of growth due to interference in crystallization which impart a charac- 

 teristic, non-fragmental structure to the rock in which it occurs. 



On the other hand, if a fissure were tilled with particles of solid matter, as 

 for instance sand, and the whole were cemented so as to form a hard dike 

 rock, it would have a decidedly fragmental character. A microscopic exam- 

 ination would certainly show that the mineral particles or grains in the rock 

 ar>- nut bounded by crystal faces or lines of growth, hut instead by lines of 

 fracture and abrasion. In the first case the crystals, whether perfect or not. 



are a- large a- they ever were; hut in the second case the -rain- are only 



fragments of broken crystals, and the term fragmental defines the charac- 

 terizing feature of t lie rock. 



From these considerations it would appear to be an easy matter to dis- 

 tinguish a rock formed in a fissure by filling it with material brought thither 

 in a liquid state, either of solution or fusion, from one produced by filling a 

 fissure with solid particles subsequently cemented ; and Buch is really the 

 case. The dike rock already described is plainly fragmental, and there can 



he no reasonable doubl whatever that the fissures were tilled with sand. 



The question at once arises, Whence came tin- sand ? It could not have 



Come from the bounding rocks of the dike upon the Bides and ends upon the 



surface, for so far as can !"• seen, they are almost always -hale-. It musl 

 have entered the fissures either from above or below. 



[f we .-uppo.-e they were slowly filled from above by loose -and brought 



thither by wind or water and dropped under the influence of gravity alone, 



the long and broad but thin grains, like scales of mica and other i v or 



less foliated mineral-, would generally lie horizontal, as they lie parallel to 



' ProfeHonr R. 1>. Irving ilencribe le " velne " on the shore ol i ike Superior, formed by 



ea from above ' il Survej Monograph V. Washington, 1883, pp 138 



