DIKES AND VEINS. 



It may, indeed, be supposed that the folding took place when the materials were in the form of clay. But 

 it is doubtful whether such great perfection in the curvatures could have been produced in clay, and retained 

 through all the subsequent changes which have resulted in a highly crystalline condition. Moreover, some of 

 the foldings in this rock have an angular sharpness which we have never soen in clay, as in the subjoined 

 sketch, in Fig. 5, taken at the same locality ; and indeed the specimen exhibited on Fig. 4 shows to the 

 eye a multitude of such serratures, too minute to be marked on the drawing. They seem to be the result of 

 strong pressure on fine folia of rock, having somewhat of stiffness, so that a lateral force would crumple it 

 up rather than produce regular curves. 



Some writers (e. g. Tyndal on the Glaciers of the Alps, p. 9) maintain that all such foldings of the 

 solid rocks might result from "ages of pressure" upon them without softening. But it is not mere cur- 



Flo 5 vature that is to be explained, but crystallization ; 



and this could take place only where the mass 

 was soft enough to allow of a movement of the 

 molecules. 



4. Some of the phenomena of granite veins and 

 trap dikes are explained with difficulty, unless we 

 suppose a plastic or semi-plastic condition of the 

 rocks through which they pass. 



Some of the granite veins that have been usually 

 regarded as veins of injection, are so tortuous that 

 the idea of their having been filled in their present 

 form by the intrusion of melted rock, is absurd. 

 For the projecting films of rock, sometimes not 

 more than a quarter of an inch thick between the folding of the vein, must have been broken off. We shall 

 give one or two sketches of such when describing granite, though such sketches convey no adequate idea of 

 these cases. We conceive that after these veins were introduced into the rocks in the wet way rather 

 than by injection, the folia were crumpled up so as to produce extreme tortuosity, and this would require 

 plasticity. 



Trap dikes are sometimes introduced into such rocks as the schists and gneiss, and where they are dislo- 

 cated the broken extremities are so implanted in the folia as could have been done only by materials in a 

 semi-plastic condition. But without drawings we cannot make this point intelligible, and we can only 

 refer for an example to the Elementary Geology above mentioned, as illustrated by its 142d figure. 



The great number of veins that sometimes form a delicate net work, can be explained only by a plastic 

 condition of the rocks, allowing the veins to be segregated or injected. This is finely illustrated by the veins 

 of calcite interlaced in limestone and clay slate along the shores of Lake Champlain. The pebble shown 

 on Fig. 226 will give an idea of such veins. The beautiful brecciated red marble wrought in the vicinity 

 of Burlington also clearly indicates a plastic condition of the base. The red angular fragments seem to 

 have floated in the matter of the veins. 



5. We have found striking examples where the pebbles of conglomerates have been elongated 

 and flattened so as at length to be converted into the silicious folia of the schists, and the cement 

 into the mica, talc and feldspar. 



This we regard as the most decisive evidence we have met of the former plasticity of 

 all the schists and of gneiss. But as we have never seen any description of such meta- 

 morphoses, and it is only till within a few days that we have become fully possessed of 

 the facts (October 1860), and moreover other geologists may demur to our conclusions, we 

 deem it necessary to go into greater detail than would otherwise be necessary. In a Report 

 on the Geology of Massachusetts made in the year 1833, a singular conglomerate was 

 described near Newport, R. I., " composed of elongated rounded nodules of quartz rock and 



