334 



" El vans are traversed in all directions by joints dividing them into 

 irregularly-shaped blocks; in some cases these are filled with schorl, 

 while in others the filling material is a ferruginous or felspathic clay. 



"Throughout the principal mining districts of Cornwall the general 

 bearing of the elvan-courses is a few degrees north of east, and they 

 are therefore approximately parallel with the majority of the most 

 productive tin and copper lodes. In other parts of the county elvans 

 are sometimes found running nearly north and south, thus nearly 

 coinciding in direction with the cross veins occasionally yielding lead 

 and iron-ores." W 



Mr. PHILLIPS gives three analyses of the E. and W. elvans (quartz- 

 felsites). 



I. H. in. 



Si0 2 72-51 72-82 ... 71-46 



AL>0 3 ... 13-31 ... 15-12 ... 15-38 



Fc 2 3 ... tr. ... 1-75 ... -30 



FeO ... 3-87 ... tr. ... 2'27 



MnO ... -62 ... tr. ... tr. 



CaO ... -60 ... -52 ... -47 



MgO ... 1-52 ... 1-06 ... -22 



K.,0 ... 6-65 ... 6-25 ... 5'51 



Na 2 ... -43 ... -51 ... 2'79 



Fl ... tr. 



H 2 -60 2-29 ... 1-70 



100-11 100-32 100-10 



Sp. Gr. 2-62 2'64 2-65 



I. "Elvan" dyke, Sydney Cove, Pra Sands, near Marazion. The rock from the centre 

 of the dyke is rendered conspicuously porphyritic by the occurrence of large crystals of white 

 and pink felspar. Quartz, mica, tourmaline and pinite also occur. Under the microscope the 

 quartz is seen to be sometimes idiomorphic, sometimes in the form of more or less rounded grains 

 with inlets and inclusions of the ground-mass. The porphyritic orthoclase is sometimes 

 surrounded by a border of micro -pegmatite, and sometimes bounded by sharp crystallographic 

 outlines. The pinite often shows the form of cordierite. It is an aggregate of green or 

 yellowish -green micaceous flakes which give the yellows, reds and blues of the first order under 

 crossed nicols. The ground-mass is micro-crystalline, so that the rock would be a pinitiferous 

 micro-granite in ROSENBUSCH'S sense. Flakes of mica are scattered through the ground-mass. 

 The margin of the dyke is less -markedly porphyritic. It is compact and sometimes banded. 

 The ground-mass is crypto -crystalline. Idiomorphic tourmaline occurs in the ground-mass. 



II. Dyke in granite of Tregoning Hill. The porphyritic constituents are the same as 

 those of the last-mentioned rock. The ground-mass is crypto-crystalline according to Mr. 

 PHILLIPS. 



III. Dyke in Killas near Hayle. "Its general colour is a dull bluish-grey; it encloses 

 a few distinct crystals of white felspar and occasionally small nests of flaky graphite. The 

 amount of porphyritically embedded quartz is exceedingly small " (PHILLIPS). 



The ground-mass of the rocks now under consideration is generally micro- or crypto- 

 crystalline ; rarely granophyric. The rocks therefore are mostly micro -granites or felsophyres. 

 Typical granophyres have not as yet been observed. 



Wales. The Dimetian rock of St. David's is associated with felsites 

 and quartz-felsites which often show pseudo-spherulitic and spherulitic 



(1) These north and south "elvans" arc not, at any rate as a rule, quartz felsites but 

 " mica- traps," a totally different class of rocks. They will be described in a separate chapter. 



