50 ME. A. HAEKEE ON THE GEOLOGICAL [Feb. I906, 



of the pitchstone, if it coincided with an old surface of erosion. 

 The actual form, as shown in the same plate, is very different. 

 Despite undulations, the junction is always a clean-cut one. The 

 undulations may often be seen to correspond with the places where 

 the pitchstone-base passes from basalt to dolerite ; but they 

 show only gentle curvature, never abrupt change of direction. In 

 this the junction agrees with an intrusive one, and differs from 

 the erosion-form which is a necessary part of the river-valley 

 hypothesis. 



The steep inclination of the base of the pitchstone in certain 

 places has already been remarked. Exceptionally, as on the north 

 side of the Bidein-Boidheach section, it seems to approach the 

 vertical. But it is further to be noticed that a steep slope is some- 

 times found in individual sheets of the complex, which on the 

 extrusive hypothesis should represent separate lava-flows. The 

 most conspicuous example is the eastern precipice of the Sgiirr 

 proper (see PI. III). Here the base consists of a sheet of felsite 

 some 30 feet thick, with parallel upper and lower surfaces, which 

 dips southward at an angle of 40°. It is clear that a thin lava- 

 flow could not rest in such a posture as this, and examination of 

 the felsite leaves no doubt that it is intrusive. It truncates 

 obliquely the columns of the overlying pitchstone; and, when 

 followed down the southern slope, it is seen to leave the base, 

 cutting into the pitchstone itself and running out into two tapering 

 tongues. 



Other sheets of felsite, which are numerous on the southerly face 

 of the eastern part of the ridge, show in different cases indications 

 of intrusive relations towards the pitchstone. A small one near the 

 base passes into a nearly vertical dyke cutting up into the pitchsone. 

 Most of the sheets have an approximately horizontal position, but with 

 a wavy course and tapering form, 1 and bifurcation may be observed 

 in several places. The felsites do not share the regular columnar 

 structure of the pitchstone, but often truncate the columns obliquely, 

 while they have a less pronounced and ruder jointing perpendicular 

 to their own contact-surfaces. These circumstances seem to afford 

 satisfactory evidence of intrusion. 



We have remarked, however, that there are also gradual trans- 

 itions between felsite and pitchstone; and it is of interest to note 

 that this relation is sometimes to be observed in connection with 

 sheets of felsite which still give evidence of being intrusive in the 

 pitchstone-mass. The sheet first mentioned, at the eastern base of 

 the Sgurr, is probably an example of this, for, where it bifurcates 

 towards the south, it seems to lose itself gradually in the pitchstone. 

 In several other cases, which can be more closely scrutinized, such 

 behaviour is quite manifest. A sheet which sharply cuts the pitch- 

 stone can be followed along its length until the line of demarcation 

 is lost, and the felsite merges into the general body of the pitchstone 



1 See Sir Archibald Geikie's sketch, Quart. Journ. Geol. Soc. vol. xxvii (1871) 

 pi. xiv, fig. 3. 



