METAMORPHIC ROCKS. 
439 
CHAPTER VIII. 
METAMORPHIC ROCKS, AND THEIR CONTENTS. 
INTRODUCTORY REMARKS. 
Under this head, it is designed to describe such rocks of the First geological district as 
have not yet been described, and of which there is demonstrative evidence, or such as renders 
it highly probable, that they were originally sedimentary rocks, but have since been altered 
in their character, so as to change them into such rocks as have usually been called primary. 
Granite, gneiss, hornblendic gneiss, sienite, and the plutonic rocks generally, are not included 
in this class, but will be discussed in another chapter. 
In the preceding chapter, the Taconic rocks are considered as altered rocks of the Cham¬ 
plain division of the New-York system ; but in those, the change is not so great as in the 
Metamorphic rocks that are now to be described, and which are considered the same, only 
still more altered by metamorphic agency. When describing the rocks of the Hudson valley 
in 1840,* I offered the opinion that those “ in the eastern parts of the counties of Washington, 
Rensselaer and Columbia, gradually change in their aspect, becoming more shining, more 
talcy, chloritic and plumbaginous, more traversed by veins and nests of quartz, and assume 
more the aspect of metamorphic rocks. The rocks of the Hudson valley dip eastward, with 
a few local exceptions, at a high angle, and seem to dip under the rocks we have been accus¬ 
tomed to call primary ; and those rocks on the eastern boundary of the valley, that from their 
position would seem to be of more recent formation, approach in characters to those called 
primary, and finally become blended with them. I feel inclined to consider them metamor- 
phic, with intrusive rocks inter stratified. Quartz in veins we know is found everywhere in 
that region, and in many places the rock is permeated by it in every part like a sponge filled 
with water, and it is evidently an intrusive rock. Again, the limestones are in places a crys¬ 
talline, white, or clouded marble ; in others a compact limestone. Again, granite and quartz 
veins are very common in the mica slate, and talcy mica slate, of the Green mountain range 
in Vermont and Massachusetts, adjoining the slate range A of the Hudson valley. The 
granular quartz of Bennington (Vermont), and which occurs in Dutchess county (New-York), 
• Vide Fifth Annual Geological Report of New-York, 1840, p. 93, 94. 
