and their Contact Zones. 23 



apparently containing fluid, and also very minute colourless 

 rectangular crystals which I could not otherwise determine. 



The nodules contained in the ground mass have the same 

 constitution, but their structure is exceedingly minute. 

 When the edges of the slice are very thin it is possible to 

 make out that they are composed of minute rounded plates of 

 some colourless mineral, having a micaceous structure. This 

 mineral is unaffected by hydrochloric acid. In the ground 

 mass of the nodules the magnesia mica is also developed in 

 isolated aggregates of slightly overlapping laminse, which are 

 irregularly rounded when seen in the direction of the pris- 

 matic axis, but irregularly rectangular across it. They are 

 dichroic, and in several nodules which I examined I found 

 them arranged with the prismatic axis parallel to the shorter 

 diagonal of the nodules.* With these there are also long 

 and narrow flakes of muscovite, and, as before, I found here 

 minute flakes of it embedded in the magnesia mica. In the 

 ground mass there are many minute black opaque and 

 amorphous masses which are probably a form of carbon. 



A most interesting, and at the same time puzzling, obser- 

 vation is the frequent occurrence of spheroidal or elliptical 

 cavities, having all the appearance of vacuities — namely, a 

 wide, dark margin and a central spot of light. I have felt 

 much doubt whether these vacuities may not have resulted 

 from imperfect mounting ; but as I observe them in almost 

 all the nodules, I have concluded that they may be natural. 

 According to some measurements they are *0005 inch in 

 diameter. I am unable at present to account more fully for 

 them. 



The nodules are portions of the rock mass which have not 

 undergone the complete change to which the remainder has 

 been subjected. They are less crystalline than the ground 

 mass.f 



Classification of the Schists. 



The examples of these rocks are so few, being only taken 

 from the nodular argillaceous and the quartzose members of 

 the group, that few words are required. An examination in 



* Further examination has shown me that by digestion with warm hydro- 

 chloric acid the brown pigment (iron) is removed in a great measure from 

 these nodules. The dichroic mica becomes perfectly colourless, and it only 

 differs in form and in its small optical angle from muscovite. 



f Professor Kosenbusch points out similar changes. See Die Steiger 

 Schiefer, #c, Strassburg, 1877, p, 256. 



