'2-- C. R. VAN HISE — 1'KI-X AMUUIAN OF THE BLACK HILLS. 



hi- field studies prove conclusively, as it seems to me, that genuine crystalline 

 schist < have developed from clastic rucks at Newport, Rhode Island, and 

 East Wallingford and Plymouth, Vermont. Not only is this true, but in 

 general his conceptions as to the manner in which the change occurred show 

 great insight. Many of his figures are almost identical in ideas with the 

 figures published <>t'the well-known schistose conglomerates of Norway and 

 Germany, more recently described. 



The pebbles of the Vermont conglomerates are mainly of quartz. Hitch- 

 cock could not believe, as was maintained by Tyndall,* that so rigid a 

 substance a- quartz, however great the pressure to which it was subject, could 

 suffer internal movement and retain its strength. That silica is so readily 

 and extensively transferred in rocks he hail no means of knowing: hence 

 he was driven to explain the presence of the distorted quartz pebbles by 

 supposing that they represented residual silica from silicates. We now 

 that both Hitchcock and Tyndall were in part right and in part wrong. 

 The process of elongation of quartz is analogous to hut not like the flow of 

 ice in a glacier. The distortion is chiefly accomplished by fractures and 

 revelations, the quartz remaining rigid ami solutions being present to serve 

 as a carrier of silica : whereas the substance of a glacier itself is alternately 

 liquid and solid. It will he noted that for this metamorphism a high degree 

 .if heat is not requisite, as is commonly assumed. The temperature of hoi 

 springs is certainly sufficient, hut it is not asserted that a higher temperature 

 was not actually present, although it i> manifest that no such heal and 

 pressure obtained as would he requisite to render quartz itself in any degree 

 plastic. The possibilities as to the plasticity of many rock-, under ordinary 

 ( litions as to heat, when brittle quartz is found to he capable to a certain 



extent offlowage, are very suggestive. 



Aiiothn- line of study presents itself in considering these squeezed con- 

 glomerates. I; may be assumed in general that the matrix bas been elon- 

 gated a- much as the pebbles. By taking many measurement-, of normal 

 erosion pebbles ami thus getting the ratio between their longer and .-holier 

 diameters, and doing like work with the pebbles of the same composition and 

 magnitude in conglomerates which have keen subject to dynamic action, we 

 would lie able to get an approximately reliable quantitative measure of the 

 amount that the beds have keen diminished in thickness by the mechanical 

 action to which they have been subject. This has not been done with the 

 Black Hills rocks, but it i- -ale to say the diminution in thickness of the 



original bed- i- very < siderable. 



Tin Mica-slates and Mica-schists.- The slates, quartzites and conglomerates 

 cur in a broad belt in the ceuter of the pre-Cambrian area, the conglnm- 

 :•• - being more largely kuown to the east. Passing north or south from this 



Gl Ips, l"li W7. 



