280 ON THE GEOLOGY AND 



The whole country to a great distance, is com- 

 posed of what may be considered as sienite, having 

 more analogy to that species of rock, than to any 

 other hitherto described ; this is, we think, the latest 

 of the well crystallised formations of the primitive 

 class ; the only primitive rock of later formation being 

 the clayslate. This constitutes the great sienite for- 

 mation of our country, and appears to extend, so far as 

 we have obtained information, from the Hudson* to 

 Virginia. It is in many places covered by transition 

 limestone and old red sandstone. 



This sienite is composed of feldspar and amphi- 

 bole in grains, which vary much in size, from micros- 

 copic particles to crystalline masses of several inches 

 in diameter. 



What particularly distinguishes this sienite from 

 all others, is the frequency and abundance of quartz 

 which is always in the hyaline state, and appears to 

 form an essential constituent of this rock — occurring 

 like the preceding minerals in various sized grains. 

 The quartz is disseminated throughout this rock, 

 without aiTccting any particular disposition, such as 

 is observed in granite, for which it has often been 

 mistaken. The relative proportions of the two first 

 mentioned ingredients vary much ; in some places, 



* This formation appears upon the Hudson, at the upper part 

 of the Haverstraw sea, and terminates about four miles above 

 West Point, presenting- a similar series of subordinate rocks, 

 and also containing some of the same minerals, such as Chon- 

 drodite, Black Spinelle and Oxidule of Iron. The direction of 

 the rock is North 2o° East and its dip East 80°. 



