240 



ME. H. H. THOMAS ON THE 



[Aug. 1909, 



It will at once be seen from Table I tbat, although there is a 

 general similarity in the mineral contents of the various divisions 

 of the New Red, it is possible to observe radical differences, which 

 become more apparent the more closely the rocks are studied. 



The distribution of garnets shown in Table II (below) brings out 

 clearly how, not only various horizons in the New Red, but also the 

 same horizons in different districts, may exhibit special mineralogical 

 characteristics. The same point may be urged with reference to the 

 distribution of staurolite, tourmaline, and several other minerals. 



Table II, illustrating the vertical and horizontal distribution of 

 garnet in the New Red rocks. 





South 

 Devon 



Central 

 Devon 



South 

 Somerset 



Bridgwater 

 District 



Upper 



Marls & 



Sandstones 



















Pebble 

 Bed 







Lower 

 Marls 











Lower 

 Sandstones 

 & Breccias 





There are, however, a certain number of mineral species which 

 are present in almost every sediment, and thus by themselves throw 

 but little light on the origin of a deposit ; at the same time, the 

 relative abundance of any mineral at different localities on the 

 same horizon is most important. 



To sum up the results expressed in the two tables given above, 

 it may be stated that the Lower Breccias and Sandstones are fairly 

 constant as to their mineral contents. In the extreme south of 

 Devon they approximate in composition more closely to the southern 

 part of the Pebble-Bed : for staurolite is common, and fluorspar 

 also occurs, in both. They contain, and are characterized by, an 

 unusually large quantity of iron-ores, brookite, rutile, felspars, and 

 felsitic material, but above all by the abundance of blue acicular 

 tourmaline, either free or embedded in quartz. Most of the 

 quartz gives uniform extinction, and there is, in South Devon, a 

 total absence of garnets. In Somerset, however, garnets are 

 fairly common in the Lower Sandstones around the inliers of 

 Cannington and Quantock. 



In the Lower Marls much more of the quartz shows undulose 



