﻿238 Scientific Intelligence. 



schists constitute isolated ridges in what may be called a sea of 

 limestone, and the four limestone belts blend with one another 

 around the ridges; even the most western blending in some 

 places with the eastern. Thus there is oneness in the system of 

 rocks. 



The oneness in system and origin is manifested in oneness of 

 gradation in metamorphism, as has been above indicated. The 

 increase in the degree of metamorphic action on going from west 

 to east is extremely gradual. It is just such as should follow 

 from variation in intensity of metamorphic action — that is, varia- 

 tion in the degree of heat, moisture and pressure concerned. 



Such facts give augmented force to the evidence that there 

 has been no general overthrust. The only rock in the series about 

 which there is any question as to its relative position is that of 

 the quartzyte which occurs here and there over the eastern half 

 and on the eastern margin of the area. 



The facts as to oneness of system and metamorphism show also 

 that the fossils have a profound bearing on the whole Taconic 

 system, although found only in the feebly crystalline rocks on the 

 west. 



Fossils are not to be looked for in coarse mica schist or gneiss, 

 or in coarsely crystalline limestone. The more or less heated 

 siliceous and alkaline waters at work in metamorphic action tend 

 to dissolve away calcareous fossils. Moreover most shells have 

 thin walls, a large part not over a fiftieth of an inch thick ; 

 and hence, if the crystalline grains made by metamorphism are 

 even a. thirtieth of an inch across, the shell may disappear. 

 Crinoidal stems and disks are among the most persistent of fossils 

 because of their thickness. The shells of Canaan, as Professor 

 Dwight states, sometimes show that under the metamorphic 

 action they were softened and mashed to pieces, coils unwound 

 and left in bits. The columns of a Solenopora are less than 

 ^J-^-th of an inch in diameter, and should be obliterated with the 

 slightest change. 



Further, hydromica schist, even when fine-grained, is a true 

 mica schist — though containing four or five per cent of water and 

 usually some chlorite — as shown by chemical and microscopic 

 analyses. A hydromica schist is in fact as much a metamorphic 

 rock as a mica schist, although the gradations may be indefinite 

 between it and an argillyte. From Mr. Van Hise's investigations 

 (This Journal, xxxi, 453, 1886), we have learned that mica may be 

 made out of feldspar by metamorphic change, and thus we are 

 relieved of the mysterious fact that mica schists had no mica 

 sediments as predecessors. Hence, from the constitution of mica 

 and orthoclase, a sediment consisting of detrital orthoclase and 

 clay (kaolin or decomposed feldspar), half and half or thereabouts, 

 may, under heat (with moisture) and pressure, be converted into 

 a hydromica schist, or else a mica schist, containing about the 

 usual proportion of quartz, this quartz being the silica set free ; 

 and more quartz, if the sediment contained quartz grains. And 



