518 MR. A. HARKER OX ROCKS 



must have been a considerable amount of unindividualized base. 

 The general characters of the groundmas 

 the normal basalts than to the andesites. 



The general characters of the groundmass bring; the rock nearer to 



3. Rhyolitic Rocks. 



The normal rhyolitic rocks do not differ in any marked way 

 from those underlying and intercalated in the Conisto7i Limestone 

 of the Lake District, Both lavas and ashes are found. The lavas 

 are not conspicuously porphyritic, though little felspars are often 

 scattered through the rock : these, as usual, are plagioclase. The 

 rocks show a generally compact appearance, light grey or cream- 

 coloured when not stained by iron oxide. The groundmass has 

 probably been to a great extent glassy, when the rocks were fresh, 

 but this has not always been the case. A rock from the beck north 

 of Keisley, for instance, has a microlithic character. 



The specimen just mentioned [919] shows a kind of flow-breccia- 

 tion, which I believe is not an uncommon feature. Very similar 

 appearances are seen in some of the Caernarvonshire rhyolites. In 

 the Keisley rock the fragments of the original lava, probably the 

 broken-up crust of a couUe, are divided by a matrix, or system of 

 irregular branching veins, which makes up quite half of the whole 

 mass. This matrix seems at first sight to consist of rather finel}" 

 crystalline quartz ; but closer scrutiny serves to detect in some of 

 the clear grains the rectangular outline and the twinning of felspar. 

 This matrix, therefore, must be regarded not as an infilling of vein- 

 quartz entirely subsequent to the formation of the rock, but rather 

 as an inflowing of the highly acid mother-liquor from which the 

 earlier portion of the rock was formed, and so as representing onlj- 

 the latest phase in the consolidation of the lava. In one place the- 

 matrix contains an amygdaloidal cavity, some twentieth of an inch 

 in length, on the border of which crystallization is rather better 

 developed, and the felspar-twinning of some of the crystal-grains- 

 is well seen. The slide shows some genuine vein-quartz which 

 occupies little cracks traversing the microlithic fragments and their 

 matrix alike, and the contrast of these with the latter can be easily 

 observed. 



Other rocks in the neighbourhood of Keisley are ashes, and one 

 from Harthwaite Beck is a vesicular andesite with finely micro- 

 lithic ground [1283]. It is noteworthy that here, as elsewhere 

 among our Ordovician lavas, a vesicular structure is much rarer in 

 the rhyolites than in the andesites. 



A well-marked type of acid lava is exemplified by a specimen 

 from Gregory Hill near Dufton, " the second rhyolite below the 

 Keisley Limestone.'"' A slide of this rock in Prof. Mcholson's 

 collection [L. D. 28] shows a groundmass enclosing a few scattered 

 felspars, which, when not too much decomposed, give faint indica- 

 tions of twin-striation. The ground has in natural light a mottled 

 appearance owing to numerous clear spots, with a tendency to 

 rounded outline, from which some dusty material seems to have 



