144 PROCEEBINGrS OP THE GEOLOGICAL SOCIETY. 



from vents which had previously emitted andesites, so that the 

 l)resent plug may represent only the later and more acid protru- 

 sions. Of the more basic bosses, besides the Castle Head, there is 

 the large mass forming Carrock Fell, wherein two distinct protru- 

 sions may be recognized, each of which may mark the position of a 

 volcanic orifice. One of them is occupied with a thoroughly typical 

 granophyre, the other with an augite-mica-diorite, and they rise 

 abruptly out of the Skiddaw Slate. Their position within the great 

 volcanic ring round Skiddaw is such as would have been occupied 

 by important vents. 



There is a considerable number of bosses formed of more acid 

 rocks, some at least of which not improbably indicate the site of 

 volcanic vents ; two of these form conspicuous features on either 

 side of the Yale of St. John ; they consist of microgranite, and 

 rise like great plugs through the Skiddaw Slates, as well as through 

 the base of the volcanic group. The view of the more eastern 

 hill, as seen from the west, is at once suggestive of a " neck." These 

 masses measure roughly about a square mile each. With the acid 

 intrusions may possibly be associated some of the other masses of 

 granophyre and microgranite which have long attracted attention 

 in this region. Eut there is also evidence of the protrusion of 

 l)ortions of an acid and basic magma at a time long subsequent to 

 the eruption of the volcanic group of Borrowdale. The Shap 

 granite, for example, is one of these younger bosses ; it invaded 

 the area after the deposition and uptilting of the Upper-Silurian 

 strata, but before the beginning of the Carboniferous period, for the 

 lowest members of the Carboniferous series are full of detritus 

 derived from its waste. 



But it is not merely in large bosses of massive rocks, whether 

 diabases, gabbros, felsites, granophyres, or granites, that we have 

 to seek the vents which supplied the lavas and tuffs of the Lake 

 District. We cannot make out such a decided accumulation of 

 the volcanic materials in certain directions as to indicate the 

 quarters where the centres of eruption should be sought. On the 

 contrary, the confused commingling of materials, and the com- 

 parative shortness of the outcrop of the several sheets which 

 have been traced, suggest that there was no one great central 

 volcano, but more probably many scattered vents, which threw out 

 their lavas and ashes over no very wide area, but which stood 

 near enough to each other to allow their ejected materials to 

 meet and mingle. The scene seems to have been rather of the type 



