‘HARKER; ANCIENT LAVAS OF THE LAKE DISTRICT. 147 
volcanic series, and in the middle of the Coniston Limestone group 
above. The rocks are rhyolites, with a silica-percentage up to 76 or 
77 and a specific gravity of about 2°6. They are hardly ever 
conspicuously porphyritic, presenting most frequently a uniformly 
compact aspect. ften they are grey, with a rather flinty 
appearance; or, again, they acquire by oxidation and other 
changes a pink or cream-colour, and become duller in aspect. 
The flow-structure is sometimes strongly marked to the eye, and 
some of the rhyolites have quite a laminated look. Thin sections 
under the microscope suggest that the rocks have in many cases 
been largely glassy in texture, but have suffered various processes of 
alteration. One type of structure met with is the micro-spherulitic. 
Among other secondary modifications, the rhyolites appear to be 
liable to impregnation with silica, and this may even go so far as to 
convert masses of the rock into a jaspery substance. More 
remarkable are the effects observed in certain globular bodies in the 
rock, having a diameter of one or two inches or more. se se 
to be originally giant spherulitic structures, not fundamentally 
different from the minute ones seen under the microscope, but they 
st 
Some of the altered spheroids show concentric alternating shells of 
chalcedony or quartz, of a black chloritoid substance, and sometimes 
of comparatively unaltered rhyolite. In certain cases actual hollows 
are formed by the destruction of portions of the interior. The final 
Stage is the conversion of the whole into a nodule of quartz, and 
sometimes the surrounding matrix of rhyolite also becomes silicified. 
Excellent examples of these phenomena are seen on the face of 
Great Yarlside and at other places in the neighbourhood. 
€ above are only the leading characters of the main types of 
Lake District lavas. The sources which gave vent to such extensive 
Outpourings are still to be localised. The thickness of the whole 
volcanic series was estimated by Mr. Clifton Ward at 12,000 feet, 
but is probably much greater. In any case it betokens the long- 
continued prevalence of intense volcanic activity. This was heralded 
a 
lingered feebly into the beginning, at least, of the Silurian period ; 
but the important display of vulcanicity in this area is comprised 
between the limits of the great Volcanic Series as defined above, 
and this part of England may dobtless furnnish important lessons 
— the essential features of a period of igneous manifestations. 
¥Y 1891, 
