1813.] Mineralogkal Observations on Cornwall. 251 



mity of Great Britain. This clay slate possesses an unusual 

 proportion of mica; but its characters are those of clay slate, 

 and not of mica slate, as Dr. Berger has called it, misled, I 

 presume, by the quantity of mica which it contains. 



All the above described bed9, which constitute the peninsula 

 of the Lizzard, lie in a conformable position to each other, and 

 dip to the south at an angle of more than 70°, as is the case with 

 the transition slate on their north side. Hence they all lie on 

 that slate, and if produced north would cover it. We cannot 

 doubt, therefore, that they all belong to the same formation, 

 or that the whole peninsula of the Lizzard consists of transition 

 rocks. The clay slate at its southern extremity, then, is a tran- 

 sition slate ; and indeed it possesses all the characters of it. The 

 only circumstance which could have prevented preceding ob- 

 servers from drawing this very obvious conclusion, is the exist- 

 ence of serpentine in the district, which has been hitherto con- 

 ceived to belong exclusively to primitive rocks; but as the 

 diallage rock, which is so intimately connected with serpentine, 

 and so frequently accompanies it, had been already found as a 

 transition rock, mineralogists were naturally led to expect that 

 serpentine would occasionally occur in the same position. The 

 occurrence of transition serpentine, then, in Cornwall, is no 

 more than a fact which we were previously led to expect. 



The soap rock which occurs at the Lizzard is found in the 

 serpentine. It is said to constitute a vein ; but after examining 

 the supposed vein with some attention, I confess that the soap 

 rock did not appear to me to constitute a true vein, but to be 

 merely a portion of the serpentine itself altered by the action of 

 water, or some other cause. It exhibits the same colours as 

 serpentine, onry they are all lighter ; and it lias altogether so 

 much of the same aspect, that I am tempted to believe that its 

 constituents will be found the very same with those of the ser- 

 pentine— an i xperiment which I shall certainly try, if my speci- 

 mens reach me from Cornwall in safety. 



V< ins of schorl are said to occur in the clay slate near the 

 Lizzard. Utile.-,.-, flu y constitute what is called b}' Mr. Jameson 

 the schorl rock of Cornwall, I know not where to find it ; as I 

 neither saw nor heard of any rocks of schorl in the county (ex- 

 cept in minute quantities), though I was on the look out for 

 them during my whole stay in the county. 



I think there is reason to doubt whether the greenstone and 

 ii 1-par porphyry reach to the west coast of the peninsula ; for at 

 Mullyan, which I conceive to be nearly opposite to the southern 

 coiiniieiici mi nt of the greenstone on the east coast, I found a 

 lit d of clay slate. As it became dark soon after I passed 

 Mullyan, J had no opportunity of seeing the structure of the 

 < (iiintiy on my W»y north to I lelston ; but as the country round 

 toil il ■ 'i is composed of clay slate, we have little reason to 



