SOME NORTH-OF-ENGLAND DYKES. IOa 
temperature ranging between 220° and 240° Fahr. for 24 days; at 
the end of that time the tube was opened under water and the 
contents filtered off. The insoluble portion was washed off the 
filter and treated with a ten-per-cent. solution of caustic potash to 
remove deposited silica. The washed residue was dried and heated | 
to near redness and weighed. It was again ground down for 4 
long time in the agate mortar, and an aliquot part sealed up once 
more with hydrochloric acid and exposed for fourteen days as before. 
It was found that in the second treatment only about 16 per cent. 
by weight had been dissolved, which seemed to afford good evidence 
that what remained was quite insoluble in hydrochloric acid under 
these conditions. The quantitative results obtained were that 39°33 
per cent. of the Ayton rock was insoluble in hydrochloric acid, and 
that on determining the silica in this residue it was found to amount 
to 77°66 per cent. These results were verified by repetition. 
After the analysis of the Armathwaite specimen had been com- 
pleted, it was thought desirable to subject a portion of this rock 
also to the foregoing treatment. This was done; but the time of 
exposure was extended to 31 days, and the temperature was raised 
to 316°4 Fahr. Under these conditions several tubes were lost by 
explosion. ‘The insoluble residue amounted to 35°57 per cent. It 
was analyzed and furnished the following :— 
SIU Crs Ha) pee eR yee 25°17 70-76 
islltuneaHh ei eee 6 oe 3°89 10°93 
Ferric oxide . 1:28 3°O9 
ime 6. hadvcloe 7 3°29 
Magnesia ...... 1°50 4°21 
Wilicalies. 23.2.3 44 2°56 Kee 
35°57 100-00 
Mr. Anson examined portions of the insoluble residue from time 
to time, duplicate tubes being prepared for that purpose, and in 
the final residues he found the doubly refracting matter to be re- 
duced to a minimum. 
Deducing an opinion from the almost total insolubility of these 
residues, their highly acidic character, and the absence of any 
marked amount of doubly refractive matter, I think we may regard 
them, at any rate approximately, as representing the base of the 
rock. This view is much strengthened by the results from the 
Armathwaite specimen which Mr. Teall handed to me as a rock 
with “a true glassy base ;” nothing could much better represent a 
glass than the composition of this residue. 
It may be of interest to state that in these tube-experiments it 
was conclusively proved that the hydrochloric acid took up none of 
the silica from the decomposable silicates, but that the whole of it 
was deposited in a pulverulent form in the tube, and was filtered off 
with the insoluble residue, from which it was, of course, separated by 
the treatment with caustic potash. This is a point which I have 
not seen noticed previously. 
Q.J.G.8. No.158. @ 
