1918] Davis: The Radiolarian Cherts of the Franciscan Group 321 



cherts are known to cover an area of at least 2000 square miles in 

 this part of Scotland. Immediately overlying the Arenig cherts the 

 Llandeilo division begins with radiolarian cherts and mudstones, 

 which are conformable upon, and pass down into, the cherts of the 

 Arenig division. The combined thickness of these cherts varies, in 

 different parts of the region, between 70 and 200 feet. Overlying 

 the lower Llandeilo cherts come graptolite shales and tuffs. Near 

 the upper part of the Llandeilo division another horizon of radio- 

 larian cherts is found, in certain parts of the region. This upper 

 chert horizon is only a few feet thick, and consists of gray cherts 

 with gray and orange colored mudstones, containing abundant vol- 

 canic ash. Above this there are occasional cherty ribs or bands, con- 

 taining radiolaria, in alternation with black graptolite shales. 



The color of the cherts varies considerably. Light and dark 

 green, sometimes bright green, gray and dark gray, black, blue, red 

 and chocolate colors are mentioned. The most common colors seem 

 to be green and gray. They are hard, compact rocks with a hackly 

 fracture. They are traversed in all directions by microscopic cracks 

 and fissures. These are filled with quartz which is frequently stained 

 by a dark substance, probably manganese. 



In detail the bedding is somewhat irregular. Part of the cherts 

 occur well banded in definite strata. Others consist of layers of 

 elongated and flattened nodules, which occasionally coalesce, forming 

 irregular lenslike beds which may persist for some distance. The sur- 

 faces of the banded cherts are often mammillated or botryoidal, and 

 vary in thickness from a few inches to a foot or more, thus causing a 

 ribbed appearance of the beds. 



The cherts are associated with fine grained gray and red mud- 

 stones which form partings between the various chert layers. Occa- 

 sionally the chert bands are interleaved with fine tuff instead of the 

 usual argillaceous parting. The mudstones may pass laterally into 

 chert by the addition of silica. These transition beds contain radio- 

 laria. 



The Arenig radiolarian cherts are closely associated with pillow 

 diabase and diabase porphyrite and with tuffs of similar chemical 

 composition. The volcanic activity represented by the rocks at the 

 base of the Arenig continued during the period of deposition of the 

 radiolarian cherts, and the cherts are interlaminated with contem- 

 poraneous bands of tuff and agglomerate. 



This volcanic series passes upwards into red, green, and gray cherts which 

 are interstratified with tuffs and breccias that clearly overlie the Middle Arenig 



