British Pillow-lavas. 203 



sack-shaped masses, globular or elongated, and varying in size. The 

 external surface of the pillows may he compact, hut in their interior 

 there are numerous cavities often concentrically arranged. First 

 described in Britain by Nicholas Whitley (1), and figured also by 

 De la Beche (2), they have become generally known through the work 

 of Teall (3), Raisin (4), Beid and Dewey (5), and their importance 

 among the Palaeozoic eruptive rocks of Britain is now well established. 



Although pillow- structure is rarely completely absent, it may not 

 be well exhibited in every exposure of these rocks, especially when, 

 as in Cornwall and Devon, they have been exposed to earth-movements 

 and folding. On the other hand, pillow-structure may make its 

 appearance in rocks that do not strictly belong to this group. Thus 

 it is sometimes seen in the Carboniferous basalts of the Lothians and 

 Fife (6), though not well developed. 



The name spilites was given to rocks of this class by Brongniart (7) 

 in 1827, and has recently, in accordance with the usage of Continental 

 petrographers, been adopted by the Geological Survey to designate the 

 Carboniferous, Devonian, and Ordovician pillow-lavas of Devon and 

 Cornwall. It was first used in English by Bonney (8) for certain 

 rocks in Jersey that seem to be properly included in this group. 



In addition to the pillow-structure there are certain characteristics 

 that mark the spilites of Great Britain. The first of these is that they 

 are as a rule very completely decomposed, and the second that their 

 felspars are always rich in soda. Their micro-structure varies within 

 rather narrow limits. The principal component is felspar ; next in 

 importance is augite of pale-brown colour ; remains of olivine are 

 sometimes, though not often, to be detected. Frequently they seem 

 to have contained a fair amount of glassy base, though this is always 

 devitrified and decomposed. Their felspars are occasionally micro- 

 porphyritic, and often those of the ground-mass have pointed or 

 acicular forms. A few British spilites have large felspar phenocrysts. 

 Sometimes they consist almost wholly of felspar laths with a fluidal 

 arrangement. A large number of spilites are variolitic, and in these 

 the augite may occur as irregular masses enclosing the ends of felspar 

 rods, so that the structure may be described as sub-ophitic. 



The advanced decomposition exhibited by the vast majority of these 

 rocks has led to their being classed as ' diabase-porphyrites ' ; and 

 when they are sheared they form typical ' schalsteins'. We find these 

 characters not only in the pre-Cambrian and Ordovician spilites. In 

 Cornwall the Devonian and Carboniferous spilites and schalsteins never 

 contain olivine or augite, but their ferro-magnesian minerals are com- 

 pletely replaced by secondary products (chlorite, calcite, and epidote). 



The felspar is sometimes decomposed beyond identification. 

 Dr. Teall (9) was the first to determine its real nature in the pillow- 

 lavas of the Southern Uplands of Scotland. He recognized it as 

 oligoclase, and confirmed this identification by chemical analyses. 

 We have examined hundreds of sections of spilites from Cornwall 

 and Devon, and have found that the only felspar they contain is 

 albite or albite-oligoclase. 



There are two ready tests by which this felspar can be recognized. 

 It has all indices of refraction lower than that of Canada balsam, 



