PEE-CAMBKIAN HOCKS OF ST. DAVID's. 303 



between Clegyr Foig and the coast, west of Khosson, where the 

 following section may be observed. 



The eruptive rocks thicken towards the south-west, as if the main 

 vents had lain in that direction. There are doubtless intrusive as 

 well as contemporaneously interbedded masses in the rough ground 

 between Pen-maen-melyn and Treginnis. To separate these out 

 would be a most interesting and beautiful piece of mapping for any 

 competent geologist in possession of a good map on a sufficiently 

 large scale. 



The interbedded lavas, so far as I have had an opportunity of 

 studying them, appear to present remarkable uniformity of petro- 

 graphical characters. Microscopically they are dull, fine-grained to 

 compact, sparingly porphyritic, ranging in colour from an epidote- 

 green to dull blackish-green and dark chocolate-brown. Some of 

 them are finely porphyritic from the presence of small glistening 

 surfaces which present the colour and metallic lustre of haematite 

 and yield its characteristic streak. Obviously basic rocks, they pre- 

 sent, as I have said, a close resemblance to many of the porphyrites 

 of the Lower Old Bed Sandstone and Carboniferous districts of 

 Scotland. So marked is this likeness that Mr. Peach and I at once 

 classed them as porphyrites, so far as their characters could be 

 judged of in the field. Subsequent microscopic study of them, while 

 showing that the resemblance descends even to minute details, has 

 brought to light some features that are seldom seen in the Scottish 

 examples, and has led me to class these rocks with the diabases. 



Two of the most conspicuous rocks of this class in the district, 

 those of Ehosson and Clegyr Foig, have been analyzed by Mr. Wilson, 

 with the following results. 



Analyses of Diabases from St. David's by Mr. J. S. Grant Wilson. 



Ehosson. Clegyr Foig. 



Silica 45-92 45-38 



Alumina 18-16 16-62 



Ferric oxide ; 1-18 4-06 



Ferrous oxide 9'27 8-63 



Manganous oxide 0-19 0-14 



Lime 7*19 8-19 



Magnesia 10-07 9-41 



Potash 1-78 0-71 



Soda 2-12 2-20 



Loss on ignition and water . . 4*22 4-34 



Insoluble residue 0-04 0-08 



100-14 99-76 



Specific gravity 2-96 2-99 



The strikingly basic composition of the rocks is well brought out 

 by these analyses. It may be difficult at first to believe that the 

 specimens were not taken from some modern basalts. That the lavas 



