780 T. 0. BONNET ON THE SEBPENTINE ANB 



are green -coloured and dichroic, indicating a change towards ura- 

 lite. Others, again, are full of microliths which are bright-coloured 

 between crossed Mcols, while the main part of the crystal is dull or 

 dark*. I cannot recognize any felspar in this slide, or any other 

 mineral besides the above, except that probably one or two fibrous 

 aggregates of serpentinous aspect are pseudomorphs after oliviuef . 

 The alteration indicated by the microscopic examination is con- 

 firmed by the analysis, which indicates a rather exceptional mineral. 

 A little beyond this spot the serpentine ends abruptly against a 

 basalt dyke from 1 foot to 3 feet wide, which I think has come up 

 along a line of fault. On the other side is a black tuff, rather 

 schistose, containing many fragments of limestone and porphyrite, 

 some of the latter being of considerable size, possibly even the ends 

 of small coulees. If I am right in supposing Br. J. Geikie to refer 

 to this rock, and consider it as " resulting from the dissipating in 

 situ of a limestone " (p. 528) , I must express my surprise that any 

 one accustomed to the study of volcanic rocks should not have at 

 once recognized these as included fragments. The limestone itself 

 has that peculiar, rather flinty, fracture so common when it has 

 been indurated. Except that perhaps the matrix seems rather more 

 earthy than usual, there is nothing exceptional about the tuff. 



The Porphy rites. 



Beyond this tuff we find the great mass ef porphyrite which forms 

 Pinbain Hill ; this is marked on the map as altered Silurian. It is, ' 

 however, a rock very similar in character to that at Balcreuchan 

 Port ; and as I examined the latter mass first and in more detail, I 

 will select it for description, merely stating that whatever conclu- 

 sion is true for the one is also true for the other. Both or neither are 

 of igneous origin. Dr. J. Geikie regards both as simply instances 

 of extreme metamorphism, the result of what we may call selective 

 action. I am unable to agree with him, as the following descrip- 

 tion will show. 



We descended from the high road to the water's edge at Bal- 

 creuchan Port, a romantic cove enclosed by precipitous cliffs. On 

 the shore, rather on the southern side, is imperfectly exposed a little 

 of a rather decomposed dark-coloured serpentine, apparently intru- 

 sive in a rock consisting of angular fragments of felstone in an earthy- 

 looking decomposed ground-mass. Their condition and the nature 

 of the ground — strewn here with boulders and debris — makes it 

 difficult to form a decided opinion about this rock ; but I have little 

 doubt that it is really a coarse porphyrite tuff or volcanic agglomerate. 

 Overlying this is a great mass of a hard reddish-coloured rock — here 

 fairly compact — of which the whole cliff appears to be composed. 



* Some of the least altered crystals in general appearance and in optical 

 characters approach enstatite ; I have not, however, been able to satisfy my- 

 self of the presence of that mineral, though these hardly appear to be normal 

 diallage. 



t I have in my collection a gabbro from Volpersdorf (Silesia) andabronzite 

 rock from the Kupferberg in Bavaria, which, macroscopically, seem to be as 

 exclusively composed of the one mineral. 



