766 



DYNAMICAL GEOLOGY. 



have been formed all over the globe, so as to characterize an age. 

 And as Archaean hills and mountains have continued to exist until 

 now, sediments of the same kind may have been made over and over 

 again until now, through all geological ages. A particular kind of 

 metamorphic rock should therefore not be confined to a particular area 

 or age. In regions like Scandinavia, Scotland, Labrador, Canada, New 

 England, and others similar, the original older Archaean beds still 

 exist ; and the succeeding metamorphic rocks are their direct descend- 

 ants ; and the chief difference which appears is a somewhat greater 

 preponderance of quartzose and micaceous rocks. Even as late as the 

 Triassic and Jurassic eras, the sediments laid down along the Con- 

 necticut valley, and in New Jersey, have the feldspar in most parts in 

 as great abundance and in as pure and unaltered a state as the gran- 

 ites of earlier time ; and subjection to the metamorphic process would 

 have made of much of it granite like the older, except that the mica 

 would not in all parts be in sufficient proportion. 



Again, similar metamorphic gneiss, mica schists, chloritic and horn- 

 blendic rocks, are proved by fossils to have been made in different 

 periods. The limestone region of the Green Mountains shown by 

 fossils to be Lower Silurian (p. 183), contains, in conformable beds 

 with the limestones, quartzyte, hydromica, chlorite and mica schists, 

 and various gneisses ; and the Upper Silurian region of Bernardston, 

 Mass., and Vernon, Vt., comprises conformable beds of quartzyte, mica 

 schist, various massive and schistose hornblende rocks, and quartzose 

 gneisses and syenyte, conformable with a Helderberg Crinoidal lime- 

 stone. Serpentine and mica schists occur of Triassic age, as shown by 

 J. D. Whitney, in the Sierra Nevada, and of Cretaceous age in the 

 Coast Range of California. So at Rothau, France, the rock contain- 

 ing the Calamopora spongites is a hornblende rock, containing epidote 

 and garnets. 



The repeated trituration of sediments ultimately causes the leaching 

 out of the alkalies of the feldspars. Hence, the shales of regions re- 

 mote from crystalline rocks generally contain little alkali ; and, when 

 this is the fact, the metamorphic process would not convert them into 

 mica schist or gneiss. The tendency to this leaching effect, with in- 

 creasing remoteness from the region whence the material of the rock 

 came, is illustrated in mica schist a few miles west of New Haven, 

 Conn. ; where, as stated on p. 761, the schist changes on going south- 

 ward gradually to a hornblende schist, — hornblende, unlike the mica 

 (here biotite), containing no potash. 



Pseudomorphism. — A Pseudomorph is a crystal that has a form that does not belong 

 to the material constituting it. If a crystal has the form of calcite, but is really quartz 

 in composition, it is a pseudomorph; it was once calcite, but in some way, probably 



