112 



and to have extended into the wall rock on one or both 

 sides to a distance proportionate to the width of the 

 serpentine bands. They thus belong to the class of veins 

 sometimes called exogenous or outward growing, as 



Photomicrograph of an asbestos vein, showing the irregular nature of the magnetite 



parting, and the contact of the vein with the serpentine wall-rock. X 14; 



crossed nicols. 



distinguished from those that are formed by filling a fissure 

 from the walls inward. Measurements of many veins 

 show that the proportion of asbestos to the two bands of 

 serpentine is I to 6-6, or that approximately 15 per cent of 

 the serpentine has taken the crystalline form of asbestos. 



In the Thetford or later serpentine, many of the larger 

 veins can be seen to follow joint planes in the original 

 rocks. Another class seems to have grown from fractures 

 caused by regional folding, as is indicated by their approxi- 

 mate parallelism. Fractures, produced in early stages of 

 disintegration of the rock by casting off shells from the 

 jointed blocks, give a series of crescent-shaped veins, 



