418 J. CLIFIOK WAED ON THE MICROSCOPIC STRUCTURE OF 



great thickness of lava and ash is unmixed, except quite at the base, 

 with strata of an ordinar y sedimentary character; the ashy beds 

 contain no fossils of any kind ; and although there frequently occur 

 considerable thicknesses of well-bedded material, yet the bulk is 

 either unstratified or but very rudely stratified, and conglomeratic 

 ash is almost unknown*. These facts seam to warrant the conclu- 

 sion that in the main the old Cumberland eruptions were subaerial. 

 Since a few bands of Skiddaw slate occur interbedded with the vol- 

 canic rocks at the very base of the series, it seems probable that 

 eruptions commenced beneath and among the waters of the Skiddaw - 

 slate sea, but that, either by elevation or the partial filling-up of the 

 shallow sea-bed, the eruptions soon become almost wholly subaerial. 

 In some districts there may have occurred extensive sheets of water, 

 in which much of the finer ash was stratified, but in which the con- 

 ditions were not such as to support life, at all events in any quantity. 

 Certainly one of the main centres of eruption was close to the present 

 site of Keswick; and in the low, wooded, and craggy hill called Castle 

 Head one sees the stump of an old Cumberland volcano which once 

 poured out its lava-sheets and scattered ashy material for many 

 miles. "While denudation has destroyed all semblance of volcanic 

 form, there yet remain at some little distance from the old vent the 

 broken ends of beds of lava and ash (such as those of "Wallow 

 Crag) which were once continuous upwards to a point far above the 

 present summit of Castle Head. Other centres may, and certainly 

 do, exist in other parts of the district; but that of Keswick was per- 

 haps one of the chief, and around it occur the greatest number of 

 lava-flows. 



It might be supposed that the great granitic masses in the 

 south-western and other parts of the district bore relations to the 

 volcanic rocks similar to those Prof. Geikie and Mr. Judd have 

 shown probably to have prevailed between many of the granitic 

 bosses and accumulations of volcanic rocks in Scotland. But, since 

 the wliole thickness of the Cumbrian ashes and lavas are metamor- 

 phosed around the Eskdale granite, and there seems no direct con- 

 nexion in chemical composition between the highly silicated granitic 

 masses and the someiuhat basic lava-flows, it would be rash to infer 

 that those granitic centres we now see represent immediately the 

 roots of the old volcanoes. When all the igneous masses of this 

 district have been thoroughly examined more may be said upon this 

 point. 



General Results. 



1. The microscopic examination of rock-structure teaches us much 

 —of the conditions under which the volcanic rocks originated, and 

 the order of their mineral structure. 



2. The more ancient volcanic rocks are intensely altered, their 

 original structure in some cases being probably completely oblite- 

 rated. 



* The few cases of slight thicknesses of ashy conglomerate occur near the base 

 of the series. 



