282 METAMORPHISM OF SEDIMENTS 



In the case of sandstones the contact rock is hard 

 and splintery, like a quartzite, the rock becoming harder 

 by the cementing together of the constituent grains by 

 quartz. Clear colourless felspar, probably orthoclase, 

 has in a few cases developed, but the commonest con- 

 tact mineral is epidote, occasionally accompanied by 

 lime garnet (grossular). Epidote is a silicate of alumina 

 and lime and is only formed in those sediments which 

 were originally calcareous. The presence of amygdale- 

 like bodies of epidote and quartz in the shales near 

 dolerite is very characteristic and has been noted in 

 many districts. Cavities with remarkably smooth sur- 

 faces, identical in appearance with the steam-holes in 

 lavas, were formed probably by the conversion of the 

 water held in the then soft sediments into steam, and 

 these spaces were subsequently partially filled by the 

 epidote and other minerals formed by heated water 

 vapour acting on the constituents of the surrounding 

 sediments. The epidote gives a green colour to the 

 contents of these cavities. The calcareous concretions 

 in the shales are sometimes converted into epidote, but 

 the lime-silicate wollastonite has not been noticed as a 

 contact mineral. 



The action of dolerite upon carbonaceous shale and 

 coal is very marked, and in ths Stormberg district the 

 seams have been destroyed over wide areas or else 

 rendered unsaleable through intrusive dykes and sheets. 

 The combined carbon and hydrogen of the coal have 

 been partly or wholly removed and a semi-anthracite 

 or anthracite produced. At the contact with the doler- 

 ite the coal is burnt and sometimes rendered columnar, 



