Dr. Mac Culloch on Quartz Rock. 455 



6. Coarse graywacke slate. 



7. Fine ditto. 



8. Perfectly fine and uniform clay slate, of a dark blue colour. 



It is important to remark, that the beds which are found at the 

 foot of the mountain, are of a coarser texture than those at the 

 summit, and these are probably the uppermost beds of the deposit. 

 Many of them are nearly black from the quantity of clay they con- 

 tain ; in others are found grains of mica, and in some there are 

 imbedded large fragments of clay slate and chlorite slate. 



In hand specimens a gradation may be traced from the finest and 

 most compact quartz rock, down to a perfect breccia, containing 

 fragments of slate, although the ground does not admit of our tracing 

 the sequence of the beds. 



If therefore we consider the circumstances which I have described 

 as existing in the finest quartz rock, the rounded pebbles of quartz 

 which it contains, and the gradation that may be traced in it through 

 all the series which I have above noticed, and which are generally 

 confounded under the term graywacke, we need not hesitate to 

 conclude that the quartz rock of Jura is a mechanical deposit, or a 

 rock recomposed from the fragments of older ones. I know that 

 authors have talked of primitive sandstone, and even of primitive 

 breccia, but the awkward nature of this compound renders it de- 

 sirable that we should, if possible, discard a phraseology which 

 involves a contradiction in terms. It is perfectly true that many of 

 the beds in Jura contain large tracts of a granular quartz, often very 

 pure and compact, and which from its crystalline texture might in 

 the hand be supposed a primitive and chemical deposit ; but the 

 occurrence of blunted fragments, and of rolled pebbles, suffice to 

 shew, that Uke many other rocks it possesses at least the compound 

 structure both of a chemical and of a mechanical deposit. Neither 



