CHAP. XIII 
THE LAKE DISTRICT 
231 
approaches a basalt, containing porpliyritic crystals of fresh augite instead of 
the usual felspars, and showing a grouudmass of felspar niicrohtes with sonie 
granules of augite and dispersed magnetite. This local increase of basic 
composition is interesting as occurring towards the top of the volcanic group. 
A porpliyritic and somewhat vesicular andesite, with large crystals of striated 
felspar in a dark almost isotropic gi’oundmass, occurs under the Coniston 
Limestone near Stockdale. 1 • i • 1 
i\Ir. Ward was much impressed with the widespread metamorphism wliicn 
he believed all the volcanic rocks of this region had undergone, and as a 
consequence of which arose the difficulty he found in discriminating between 
close-grained lavas and fine tuffs. There is, of course, a general induration 
of the rocks, while cleavage has widely, and sometimes very seriously, 
affected them. Tliere is also local metamorphism round such bosses as the 
Shap granite, but the evidence of any general and serious metainorphism 
of the whole area does not seem to me to be convincing.* 
With regard to the original structure and subsequent alteration of some 
of the andesitic lavas, an interesting section has recently iieeii cut along the 
road up Borrowdale a little south of tlie Bowder Stone. Several bands of 
coarse aniygdaloidal lava may there be seen interstratified among tuffs. The 
calcite amygdales in these rocks are arranged parallel to the bedding and 
therefore in the planes of How, while those lined with chlorite are more 
usually deformed parallel to the direction of the cleavage. This difference 
suggests that before the cleavage took place, not inqirobably during the 
volcanic period, the rocks had been traversed by heated water p-oduemg 
internal alteration and rearrangements, in virtue of which the vesicles almig 
certain paths of permeation were filled up with calcite, so as then to offer 
some resistance to the cleavage, while those which remained empty, or which 
had been merely lined with infiltrated substance, were flattened and pulled 
out of shape. Messrs. Barker and Marr have shown that the aniygdaloidal 
kernels had already been introduced into the cellular lavas before the intru- 
sion of the Shap granite. In the account to be given of the Tertiary 
plateau-basalts (Chapter xxxvi.) evidence will be adduced that this tilling 
up of the steam-cavities of lava may take place during a volcanic period, 
and that it is probably connected with the passage of heated vapours or 
water through the rocks. , i 1 
Though acid lavas are not wholly absent from the central and lower 
parts of the volcanic group, it is at the top that tlieir chief development 
appears to occur. These rocks may be grouped together as felsites or 
rhyolites. They probably play a much larger part in the structure ol the 
southern part of the volcanic area than the published maps would suggest, 
and a detailed survey and petrographical study of them would well reward 
the needful labour.- A fine series of felsites is mterbedded in the lower 
1 The metamorpliism of all the rocks, aqueous and igneous, around tlie Shap granite has been 
well worked out by Messrs. Marker and MaiT, Quad. Jmirn. <lcol. Soc. vol. xlvii. (1891) p. 26b, 
SerMr^V^ Rutley, “The Felsitio Lavas of England and AVale.s,” Mem- f! col. Sun. PP- 
12-15 ; also the description of Messrs. Marker and Marr, Quart. Journ. <Eol. Soc. xlvii. (1891), p. -301. 
