114 NEW YORK STATE MUSEUM 



Catskills or the White mountains ; we are looking at the very founda- 

 tion rocks of the continent.'' It is good for our imagination to 

 attempt to grasp the meaning of such statements. 



The Grenville Series 



The geologist starts with the theory that the northeastern portion 

 of North America was at one time submerged under a great body 

 of water, in all probability marine. The ancient streams that 

 flowed from the adjacent land areas into this ocean, carrying debris, 

 deposited upon the bottom layer upon layer of muds, sands and 

 gravels. A very primitive form of life existed that performed the 

 chemical change which produced limestones from the various cal- 

 cium compounds in solution in the waters. Thus there was depos- 

 ited a great thickness of sedimentary rocks including limestones, 

 sandstones, shales, conglomerates and their intermediaries. Among 

 geologists this group of rocks is known as the Grenville series. These 

 rocks have not only been largely washed away but the remnants 

 have become so changed from their original character both in appear- 

 ance and in mineralogical make-up that it is often difficult to identify 

 them correctly. 



The Limestones 



The limestones, although limited in extent, are the most con- 

 spicuous rocks remaining. They have been subjected to so much 

 heat and pressure that they usually have lost all trace of their original 

 bedding planes and are recrystallized into creamy white marbles 

 exhibiting beautifully the rhombohedral form of calcite. Usually 

 they contain small grains of dark minerals, such as the black oxide 

 of iron (magnetite) ; garnets and green silicates (pyroxenes) ; and 

 sometimes shiny flakes of " black lead " (graphite) which is prob- 

 ably the crystallized residuum of organisms that inhabited the 

 primeval sea. 



The Sandstones-quartzites 



The sandstones, members of the Grenville series, have likewise 

 been altered and changed to white quartzites, so called because 

 they are composed to a very large degree of quartz, although the 

 impure sandstones have developed other minerals in addition. 



The shales and conglomerates have been so modified that their 

 identification is a matter of great difficulty. Some of them have a 

 composition approaching that of granite (quartz, feldspar and mica) 

 while others are darker colored. In all cases the dark minerals 

 have been squeezed into parallel lines giving rise to what the geolo- 



