Vol. 4] Thelen. — Thermal Conductivities of Certain Schists. 223 



In both of these sections the calcite appears in elongated 

 rectangular sections parallel to the pencil structure of the rock, 

 but making' various extinction angles with this direction, as in 

 fig. 9. 



Fig. 9. 



The muscovite is very abundant, filling in the spaces be- 

 tween the quartz. Some sections lie in the planes of these two 

 slides, other's are apparently on edge; still others occupy inter- 

 mediate positions. These two sections would indicate the pres- 

 ence of 15 per cent of muscovite. 



The end section presents an entirely different appearance. 

 The quartz grains seem much more numerous and closely packed, 

 none of them being masked by muscovite. The muscovite here 

 appears as foliae interstitial between the quartz, and forming 

 a network of red, green and violet "Fisseln," as seen under 

 crossed Nieols. None of the characteristic elongated sections of 

 calcite appear here. 



Summary of the structure of this rock : 



The muscovite occurs in minute flakes whose planes are 

 always perpendicular to the end section of the rock. Without 

 checking by chemical analysis and mindful of the deceptive 

 masking power of muscovite, we may assume it to make up 6 to 

 8 per cent of the rock. 



The quartz makes up 90 per cent of the rock and in those 

 few sections which show one greater dimension of the fragments 

 these are generally parallel to A of the rock. But this dimension 

 has no relation to any axis of elasticity of the quartz. 



The calcite occurs in pencil shaped forms with the ratio of 

 dimensions about 1:1:1. All the long axes are parallel. It shows 

 evidences of strain. It constitutes about 1 per cent of the rock. 



A few flakes of biotite occur, also a few grains of magnetite. 



A single case of polysynthetic albite twinning was observed. 



*Cf. Eosenbusch 's Gesteinslehre, p. 501. 



