Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTERN EUROPE. 349 



central mass of agglomerate there. But that some kind of saucer- 

 shaped depression was still left above the site of the vent is indicated 

 by the curious elliptical mass of rock (e) that lies immediately 

 above the agglomerate from which it is sharply marked off. This 

 is one of the most puzzling rocks in the district, probably in large 

 measure owing to its advanced state of decay. It is dull red in 

 colour and decomposes into roughly parallel layers, so that at a 

 short distance it looks like a bedded tuff, or like some of the 

 crumbling varieties of banded lavas. I could not obtain specimens 

 fresh enough to put its nature and origin beyond dispute. What- 

 ever may have been its history, this ferruginous rock rests in a 

 saucer-shaped depression lying directly above the agglomerate of 

 the vent. The form of this depression, like that of the Faroe 

 necks, corresponds fairly well with what we may suppose to have 

 been the final position and shape of the crater of the little volcano. 

 The rock that occupies the hollow dies out towards the east on the 

 face of the cliff, and the prismatic basalt (c) is then immediately 

 covered by the rest of the basalt-sheets of the plateau (/). On the 

 western side its precise termination is concealed by grass. But it 

 must rapidly dwindle in that direction also, for not many yards away 

 it is found to have disappeared, and the basalts (d and /) come 

 together. 



Though the decayed state of this rock does not warrant any very 

 confident opinion regarding its history, I am inclined to look upon 

 it as a deposit of much disintegrated volcanic detritus washed into 

 the hollow of the old crater when it had become filled with water, 

 and had passed into the condition of a maar. The peculiarly 

 oxidized condition of its materials points probably to long atmospheric 

 exposure, and an examination of the surrounding parts of the 

 district furnishes more or less distinct evidence that a considerable 

 lapse of time did actually intervene between the cessation of the 

 eruptions of the Portree volcano and the next great basalt-floods of 

 this part of Skye. 



That volcanic eruptions from other vents continued after this 

 Portree example had become extinct is proved by the great sheets 

 of basalt (/) that overspread it and still bury a large tract of the 

 fragmentary material which it discharged. At a later time a fissure 

 that was opened across the vent allowed the uprise of a basalt dyke 

 ((/), and subsequently another injection of similar material took place 

 along the same line of weakness (h and i). 



Before leaving this interesting locality we may briefly take note 

 of the distribution of the ashes and stones ejected by the volcano, 

 and the evidence for the relative length of the interval between the 

 outflow of the lavas below and that of those above the tuff and 

 volcanic conglomerate. Admirable sections of these deposits may 

 be traced along the base of the cliffs for a mile to the west of tho 

 vent. They thin away so rapidly in that direction that at a 

 distance of f mile they do not much exceed 50 feet in thickness. 

 At Camas Ban they consist mainly of a fine, dull-green, granular, 

 rudely stratified basalt-tuff, through which occasional angular 



