Research into the history of the old mines suggests that two mineralized fissuring systems intersect in central Devon, often carrying commercial ore-bodies. The older of these is the east-west system usually carrying copper, iron, or tin, which has been extensively worked in west Devon, but is less developed on the eastern flank of the Dartmoor granite massif.

The younger north-south series, characterized by its lead and silver ores and baryte is particularly well developed in the Teign valley. This band of mineralization can be traced from the north of Scotland through Brittany and Spain to Algeria. In Devon it appears to be divided into three main fissure bands:

1. 1. Combe Martin to Plymouth.

2. 2. North Molton to Spreyton, with a possible extension south of the Dartmoor granite.

3. 3. Molland to Newton St. Cyres and the Teign valley.

This paper describes the occurrence of myrmekite in a graphic quartz-microcline-rock from the mica-bearing pegmatites of Kodarma, Bihar, India. The myrmekite has grown out from the quartz-mierocline junctions into the potash-felspar with the appearance of having replaced the latter. The two are in optical and crystallographic continuity and, owing to the large ‘single crystal’ character of the microcline, definitely oriented sections can readily be prepared. A detailed study of these sections shows that the myrmekite has been formed by the segregation and coalescence of microperthitic albite formerly held in solid solution in the potash-felspar. Successive stages in this process of segregation can be observed.

These sections also provide evidence of the paragenetic relationship between the quartz rods, the vein perthite, and the body microcline. This is supplemented by additional evidence from graphic granites and vein perthites obtained from other localities.