and their Contact Zones. 55 



way gave a strong precipitate of chloride of platinum and 

 potassium. Those fragments which I examined under the 

 microscope by polarised light, and between crossed nicols, all 

 showed more or less perfectly the peculiar twinned and cross- 

 barred structure characteristic of microcline. 



An examination of thin slices of this rock taken from 

 near sketch section No. 1, sketch section No. 2, and sketch 

 section No. 3, showed me that in each case the same 

 peculairly twinned triclinic potash felspar formed the greater 

 part of the crystalline granular mass. In Figs. 16, 17, 18, 19, 

 I give some characteristic instances. 



The orthoclase in this rock does not offer any marked 

 peculiarities. It is quite subordinate, except locally. 



In accordance with the plan upon which I have con- 

 structed this essay, there ought to follow here an enumera- 

 tion and description of the other constituent minerals of 

 these contact schists, On attempting this description, I 

 have found, however, that it is better to sacrifice mere 

 method to the greater convenience of describing the con- 

 stituent minerals when speaking of the contact schists 

 themselves, and to do this partly from slices of typical 

 rocks, and partly from such slices in connection with 

 the detailed descriptions of the sketch sections Nos. 1, 

 2, and 3, which will follow in the next few pages. All 

 that is necessary, therefore, will be to mention now that the 

 mica of the aplite beds is a colourless or slightly greenish 

 silvery mica, having all the microscopic optical characters of 

 muscovite. In typical aplite rocks it is extremely rare, but 

 in other samples it increases so much in amount that the 

 rock becomes a form of muscovite granite. In the latter 

 case magnesia mica is frequently associated with it. 



Quartz also occurs in the aplite rocks as an essential con- 

 stituent. It occasionally contains fluid cavities with mov- 

 able bubbles. Inclusions in the felspar or quartz of the 

 aplite are extremely rare and undeflnable. 



It only remains before proceeding to speak of the contact 

 schists, to refer to the small limited zone in which is found 

 the felspathic rocks I have just mentioned. 



The contact schists fall naturally into zones, of which the 

 most highly altered immediately adjoins the igneous mass. 

 In the immediate contact of the igneous and sedimentary 

 rocks there are beds of a more or less well marked crystalline 

 granular felspathic character; on one side they all join, and 

 even are partly included in the igneous masses, and on the 



