DEFINITIONS. 453 



structure, or cleavage, the mass is homogeneous, or of the same composition ; in a schis- 

 tose structure, the ingredients are arranged in alternate layers or folia. In some of the 

 schists, however, such as the micaceous, talcose and hornblendic, the composition is essen- 

 tially homogeneous, but they are crystalline, and clay slate and shale are never so. Hence 

 we think that the definition ought to be modified so as to describe slate as homogeneous 

 and uncrystalline, and schist as heterogeneous and crystalline, with the ingredients usu- 

 ally in alternating layers. Shale is merely indurated clay, or an earlier condition of slate. 



Besides the above structural arrangements, which are confined to the stratified class, we 

 find another set of divisional planes in all the formations, stratified and unstratified. 

 These are called joints. They traverse rocks in all directions, though they show a strong 

 tendency to parallelism often over wide areas. In the unstratified rocks they are rarely 

 parallel ; but when they are so, they give to the ledges the appearance of stratification. 

 In the stratified rocks they sometimes run parallel to the strike or direction of the elevated 

 edges of the rock, and are then called " strike joints." Others cross the strike, generally 

 at a large angle, so as to divide the rock, in connection with the planes of stratification, 

 into rhomboidal or even cubical forms. 



Joints like cleavage and foliation are a superinduced structure commonly supposed 

 to be produced by the manner in which the beds are consolidated, or by subsequent me- 

 chanical force. But we shall discuss this subject under the unstratified rocks. The im- 

 portance of joints in practical operations is too obvious to require illustration, since with- 

 out them the blasting out of unstratified rocks would be almost impossible, and that of 

 the stratified quite difficult. 



The following is a list of the Azoic rocks in Vermont, which we shall describe in the 

 order below : 



GNEISS. 



HORNBLENDE SCHIST. 

 MICA SCHIST. 

 CLAY SLATE. 

 QUARTZ ROCK. 

 TALCOSE SCHIST. 

 SERPENTINE AND STEATITE. 

 SACCHAROID Azoic LIMESTONE. 



GNEISS. 



The essential ingredients of gneiss are quartz, feldspar and mica. The feldspars in the gneiss of 

 Vermont are of two kinds ; the potash feldspar Orthoclase, which is the most common, and the soda feldspar 

 Albite. No other species of feldspar has been recognized within the State. The mica is the common mica 

 or Muscovite. A large part of the gneiss of Vermont is deficient in feldspar. Those component minerals 

 are arranged more or less in folia, and the rock is stratified. The Gneiss Formation consists of gneiss and 

 its varieties, with a few associated rocks, generally in the form of beds. There are three belts of gneiss in 

 the State ; one along the axis of the Green Mountains, the second in patches along Connecticut Kiver, 

 and the third is in the southern part of the State, lying between the other two. 



