124 



DR HATCH ON THE LOWER CARBONIFEROUS 



distinguished with the lens, when the hand-specimen is held in a good light. A slice 

 examined under the microscope is seen to be made up of rather broad lath-shaped 

 crystals, colourless, of glassy habit, and showing no twin-striation between crossed nicols. 

 The crystals are either single individuals or dual twins built up on the Carlsbad type. 

 When not in juxtaposition, the intervening spaces between the lath-shaped crystals are 

 filled with microlitic felspar. No ferro-magnesian mineral was observed, but there is a 

 good deal of dusty ferruginous material present. 



Mr G. Barrow, of the Geological Survey, has kindly analysed this rock for me in 

 the Survey Laboratory at Jermyn Street : — 



Si0 2 .... 



57-50 



A1 2 3 .... 



18-89 



Fe 2 3 ) 

 FeO | 



7-51 



CaO 



1-80 



MgO .... 



1-33 



K 2 .... 



5-90 



Na 2 



5-71 



Loss on ignition 



1-70 



100-34 



Traprain Law. — This hill, which rises out of white sandstones and shales on the 

 southern margin of the volcanic area, is built up of a close-grained, dark-brown to grey 

 rock, occasionally presenting glancing cleavage surfaces of a clear, glassy felspar (sanidine). 

 Fresh-fractured surfaces have the glistening or greasy lustre already noticed in the 

 trachytes of North Berwick Law and the Bass Eock. Some varieties are speckled over 

 with dark spots, while others show a distinct banded (flow) structure, especially visible 

 in the stone quarried at Black Cove. A tendency to split into rather thin plates is also 

 noticeable. Under the hammer the stone has a remarkable sonorous ring, and small 

 fragments rattled together give a metallic clink.* 



Microscopic examination shows that the rock consists mainly of small lath-shaped 

 crystals of sanidine, arranged so as to produce a marked micro-fluidal structure. 

 Porphyritic crystals are rare, but occasionally occur. 



A bright green pyroxene is distributed through the rock in small crystals and 

 ophitic patches. It shows allotriomorphic relations with regard to the felspar. That 

 this mineral is a soda-augite is proved conclusively by the chemical analysis, there being 

 practically no magnesia present. In the few cases, however, where extinction-angles 

 could be measured to definite boundaries, they proved too high for segirine ; but the 

 presence of this mineral cannot be considered to be thereby excluded, as the association 

 of segirine with a soda-augite of similar appearance, but high extinction-angles, has been 

 recorded. 



A small quantity of iron-ore and of apatite occurs in isolated granules. 



* This property, which appears to be a characteristic of the phonolites, was noticed in the Survey Memoir (p. 52), 

 where the rock of Traprain Law is described as a " felstone (clinkstone)." 



