220 C. R. VAN RISE — PRE-CAMBRIAN OF THE BLACK HILLS. 



the Cambrian quartzite mentioned and other quartzitea in which the out- 

 lines of the original grains have uol been modified since deposition. This 

 difference is plainly due to the powerful dynamic action to which they have 

 been subject. In their transformation no evidence has been discovered 

 to -how whether any of the material has ever been highly heated, as 

 is usually assumed to be the ease in 'metamorphosed rocks. Siliceous in- 

 duration is known frequently to occur as a surface phenomenon. So far as 

 can be seen, the causes which have obliterated to a greater or less degree 

 the evidence of clastic characters are purely chemical and mechanical. It 

 is easy to see that, in the non-conglomeratic phases of rock, it' the squeezing 

 had been somewhat more intense, the proof of fragmental origin in them 

 would have been wholly obliterated. It is to be noted in this connection 

 thai the coarse conglomerates which have an unusually crystalline matrix 

 show, macroscopically, mosl strongly the deformation effect- and merging 

 together of the pehhles. 



The silica-bearing solutions which traversed the Black Sills quartzites 

 and conglomerates wen- not only capable of depositing, but, as shown, did 

 actually deposit quartz, thus preventing these rocks from becoming pulver- 

 ized during the movements through which they passed. When cracks 

 formed of sufficient size, either in the rock as a whole or in the individual 

 grains, they were at that time or subsequently cemented with new quartz. 

 At favorable moments the particles began growing, each coordinating the 

 new quartz to itself. Also in the interspaces independent quartz was 

 deposited. Consequently, while the mosl Bchistose of these rocks have now 

 become composed of angular interlocking particle- of quartz, showing little 

 or DO evidence of clastic character, they are nut less Strong than vitreous 

 quartzites which have become completely indurated without motion by the 

 growth- of the old rounded -rain- until the enlargements met and inter- 

 locked. 



The solution of silica in ruck- -given the element of time —with great 

 readiness, and its deposition as quartz in the interspaces of locks in vast 

 quantities, seem at first almost incredible; yet no one who ha- examined 

 microscopically the quartzites of our continent can doubt for a moment that 

 Buch is the fact. For the most part in ordinary quartzites the original 



iin- lie.-,.-, round and perfect a- the day in which they were deposited in 

 -a ii< I -tune-. Suppose a sandstone to he composed of spherical grains of quartz 

 of equal size, the panicle- being packed as closely a- i- geometrically pos- 

 sible, the amount of new quartz required to completely till the interspa 

 would he twenty-six one hundredth- I,'. S. Woodward I of the total -pae,-. mi- 

 ii lore than one-third of that occupied by the original grains. A- a matter of 

 tact, under natural conditions this amount has never heeii deposited, because 

 i he grains of sandstones are not spherical nor of equal Bize; because the inter- 



