PEE-CAMBEIAN BOCKS OF ST. DAVID'S. 297 



eruption. No. III. shows a considerably larger ratio of silica, as 

 was inferred on the ground from its macroscopic characters. It has 

 probably resulted from the admixture of a proportion of felsite de- 

 tritus with the predominant more basic materials ; and it may also 

 have undergone some amount of decomposition before being covered 

 up and compressed into stone. 



The occurrence of such basic tuffs in rocks of so high an anti- 

 quity is a fact of great interest in the study of the history of volcanic 

 action. But it is further deserving of attention that in the midst of 

 these tuffs there are others of a thoroughly acid character. That 

 felsitic lavas were present in the eruptive vents of the period is shown 

 by the scattered felsitic lapilli in some of the dark-red basic tuffs, and 

 by their abundance in the paler varieties. Even to the naked eye 

 some of the green, white, and yellowish tuffs are obviously composed 

 in large measure of felsitic detritus. I have been favoured by 

 M. Renard and Mr. Wilson with the following analyses of typical 

 specimens. 



Analyses of Felsitic Tuffs from the St. David? s District. 



IY. Greenish felsitic breccia, Clegyr Hill (Mr. Wilson). This 

 rock is composed of angular fragments of various felsites imbedded 

 in a greenish base. 



Y. Grey granular felsitic tuff : the last bed visible, north of the 

 bridge, over the Allan river, north from the Schools, St. David's 

 (Mr. Wilson). 



VI. Pale pinkish-white, finely schistose tuff, a characteristic 

 sample of the " Porth-lisky schists" (M. Renard). 



IY. Y. VI. 



Silica 80-59 73-42 72-63 



Alumina 11-29 12-09 16-23 



Ferricoxide 0-28 0-91 2-70 



Ferrous oxide 141 313 0-48 



Manganous oxide -. trace. 0-25 



Lime 0-52 294 018 



Magnesia 095 1-12 1*36 



Potash 2-98 1-67 3-35 



Soda 0-72 3-88 015 



Loss on ignition and water 1*96 1*28 3-04 



100-70 100-69 10012 



Specific gravity 2-55 2-74 



I have had a series of thin slices made from characteristic speci- 

 mens of the tuffs, taken from all parts of the group. It is one of 

 the most interesting series of volcanic rocks I have ever had an 

 opportunity of studying. Some of the more important details may 

 here be given. 



Many varieties of texture can be traced, from large-grained brec- 

 cias like that of Clegyr Hill to fine schistose mudstones or sericitic 

 schists like those of Porth-lisky. One of the most remarkable tuffs 



