114 MINERALOGY AND LITHOLOGY. 



Barker to be argillitic mica schist, since on analysis the specimens yielded 

 no magnesia. They were supposed to be talcoid because they had a 

 soft-soapy feel, and an appearance of talc. These schists are widely 

 distributed, while the talc schists are much more local occurrences. 



73. Serpentine [Mg^ Sr O' -\-2 H- O] 



Serpentine is not a common mineral in our state. It is found in small 

 amounts in some chloritic rocks of the Connecticut valley, and Mr. Hunt- 

 ington has observed it at Pittsburg. It is found in light green granular 

 layers or aggregations, which are either interstratified in the rocks, or scat- 

 tered irregularly through them. Again : in some of the trap rocks, serpen- 

 tine is a microscopic product which results from the alteration of olivine. 

 As serpentine, wherever found, has the microscopic appearance of being a 

 product of alteration, so, in thin sections, it has not the properties of an 

 original crystal, but its optical behavior is that of an aggregate. It gives 

 bright colors when revolved between crossed Nicols, but shows no other 

 crystalline characters. Its microscopic appearances are various, and de- 

 pend much on the mineral from which it is derived ; but no extensive 

 material for its study is offered in our state. 



74. Kaolin and Clay [AP Si^ O' + 2W O]. 



Products going under these names abound all over the state. Kaolin 

 is the mineral species to which they must be referred, although in a pure 

 state it is not common. Kaolin is formed in all granitic regions by the 

 decomposition of feldspar. In the Southern States, below the limit of 

 glacial action, where the resultant products have often remained in place, 

 there exist immense beds of soft rock composed essentially of quartz and 

 kaolin; but in our section, where the glaciers broke down and removed 

 all such decomposed materials, the kaolin is found in the lowlands mixed 

 with pulverized quartz, forming the clay beds. Under the microscope, 

 the composite nature of clay becomes very evident, yet the kaolin is 

 often seen to be in minute crystalline scales. This crystalline character 

 of the kaolin of clays was first pointed out by S. W, Johnson and Blake. 

 Kaolin is orthorhombic. That the particles are crystalline is easiest 

 seen by noting that they are double-refracting, and give colors between 

 crossed Nicols. Besides quartz, our clays contain ferruginous materials, 



