426 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 14, 



south-south-western direction, the latter approaches nearer to a 

 north-east and south-west line. In consequence, both veins should 

 unite into one, — as has actually been observed, though without in- 

 crease of the ores. Furthermore, while the first vein dips in its 

 higher parts towards the north-west, but soon changes to the 

 opposite direction at a very acute angle, and later with the constant 

 angle of 68°, the second vein steadily and regularly dips to the 

 south-west at an angle of 75°. In consequence of their respective 

 dips, both veins meet below. Therefore, and on account of their 

 respective bearing and dip, their junction is effected in a gently 

 sloping line ascending from north-east to south-west. Underneath 

 this line the lead-ore evidently is on the decrease ; and the mines 

 therefore are only explored in the higher parts. 



In reference to the geological age of both veins, the first is the 

 older, and the second the later formation. The first vein above the 

 line of junction is intersected and slightly dislocated by two cross- 

 courses, which terminate distinctly at the most regular westerly 

 wall of the second vein. Underneath the line of junction, both 

 veins, being closely linked together, are traversed and considerably 

 dislocated by the third cross-course. By means of these facts, five 

 geological periods became discernible, viz. : — 



a. The formation of the first vein. 



b. Its intersection by the first cross-course. 



c. Its intersection by the second cross -course. 



d. The formation of the second vein ; and 



e. The intersection of the united veins by the third cross-course. 



It is self-evident that such powerful and repeated changes, ex- 

 tending over only about 600 square yards, or about 1 -500th of a 

 square mile, lead to the conclusion of powerful agencies having been 

 at work, thereby seriously affecting the surrounding rock-formation. 

 And such evidence, if transferred, by way of analogy, to the many 

 unexplored square miles, offers sufficient material for geological cal- 

 culations, particularly relative to the formation of valleys of eleva- 

 tion, and the peculiar features of the Highlands in general. 



Besides the above-mentioned cross-courses, there exists at Tyn- 

 drum a cross lead-ore vein, combining the first vein with the second 

 one, from east to west. The respective widths of these three veins 

 are from 2 to 3-1- feet, from 4 to 18 feet, and from 11 to 2± feet. 

 The first vein contains solid galena ; the second one has fine-grained 

 lead-ore and silicate of lead ; the third vein contains both varieties. 

 The first vein is the most mineralized, containing mainly solid 

 quartz, several of the common spars, copper-pyrites, and blende, 

 seldom cobalt-ore, titanic iron-ore, and iron-pyrites. The second 

 vein contains many fragments of soft mica-schist, intermixed with 

 frangible quartz, copper-pyrites, and blende. The third vein par- 

 ticipates in the nature of both the others. 



The vein first named is characterized by its extent ; for it may be 

 traced through valleys and over hills for a distance of more than 

 eight miles. 



The mines of Tyndrum have, with intervals, been worked at 



