20 



He very soon discovered that while the broad rock sheets 

 succeeded and were related to one another in apparent sequence, 

 precisely as depicted by Murchison, individual parts of the section 

 gave evidence of disturbance and even inversion. At the same 

 time, well-weathered specimens of the rocks frequently displayed 

 an astonishing contortion entirely incompatible with really simple 

 relations of the greater bands. 



Attention was then focussed on the third series, the " Lower 

 Silurian," which Murchison thought to consist of three members, 

 a Lower and an Upper Quartzite, with the Durness Limestone 

 between. The first point was to ascertain exactly what the ' Lower 

 Quartzite ' consisted of, and to break it up into lithological or 

 palaeontological zones which could be mapped on the six inch, or, 

 if necessary, on the 25 inch scale. The section made out by 

 Lapworth was as follows : 



Under the Durness Limestone : 



3. The Salterella (or serpulite) grit or quartzite. 

 2. The Fucoid beds (in three divisions) 



(c) Quartzose flags. 



(b) Fucoid limestone. 



(a) Fucoid zone (flaggy grey shales). 



i. The Main Quartzite (in four divisions) 



(d) 'Pipe rock.' 



(c) White quartzite. 



(b) Tinted quartzite. 



(a) Basal (conglomerate) zone. 



The examination of the ' Upper Quartzite ' showed that it 

 invariably consisted of rocks exactly comparable with one or more 

 of these lower quartzite types, but of no others. Further, that 

 when more than one of the types existed the sequence of types was 

 that found in the ' Lower Quartzite ' either in normal or inverse 

 order. Then he was able to trace out synclines of Durness Lime- 

 stone beyond which beds comparable with some sub-division of 

 the ' Lower Quartzite ' succeeded ; sometimes as though under- 

 lying the limestones and therefore older, at other times as though 

 dipping away from it or overlying it and therefore newer. From 

 this it became evident that the ordinary rules of simple stratigraphy 

 led to two diametrically opposite conclusions as to the succession, 

 and could therefore not be relied upon to give a correct solution. 

 Finally it was found possible to start upon the ' Upper Quartzite ' 

 and, by walking along its outcrop without ever leaving it, to arrive 

 on the outcrop of the ' Lower Quartzite/ Thus the view of 

 Nicol that the ' Upper Quartzite ' had no separate existence, but 

 was merely the repetition of the ' Lower Quartzite/ was confirmed, 

 and the complicated nature of the stratigraphy demonstrated. 



