Geological Society, 37 1 



fconstitutes the principal and fundamental rock of the 

 island: in particular, the three well known conical paps of 

 Jura, of the height of 2500 or 2600 feet, are entirely com- 

 posed of this mineral. It is disposed in regular uninter- 

 rupted strata six or eight feet in th'ckness, and rising for 

 the most part at a considerable angle towards the west. 

 These strata do not appear to be traversed by veins, except 

 of quartz, nor do they alternate with any other rock. On 

 the shore, however, the dip and direction of the beds vary 

 considerably. The mineralogical composition of this rock, 

 presents several varieties. Sometimes it is extremely com- 

 pact, being made up of grains of quartz of various degrees 

 of magnitude united without cement. Sometimes besides 

 the quartz it contains felspar, seemingly in rounded frag- 

 ments, and often decomposed into clay. 



In one specimen a manifestly water-worn pebble of 

 quartz is inclosed: and upon the whole the rock may be 

 considered a kind of sandstone consisting of quartz and 

 felspar, the former in the larger proportion. In some of 

 the beds the sandstone passes into grauwacke slate by mix- 

 ture with pieces of mica slate. 



From these circumstances Dr. M. considers the quarts 

 rock of Jura as a mechanical deposit formed from the 

 fragments of older ones, and not as belonging to the Wer- 

 nerian primitive class. According to Professor Jameson, 

 however, this very rock rises from below the micaceous 

 schistus. We must therefore admit, either that the micaceous 

 schistus described by Professor J. is not primitive, or that 

 the circumstances under which the primitive rocks were 

 formed were such as to exclude at the same time the pro- 

 duction of a mixed mechanical deposition. 



T\\Q next article in this paper contains some miscellaneous 

 remarks on the geology of the island of Rona. The prin- 

 cipal rocks that here make their appearance are gneiss and 

 hornblende rock (including under the latter denomination 

 both hornblende slate and green-stone slate). Where these 

 two rocks come in contact, the gneiss is irregularly 

 curved and c )ntorted. The gneiss is traversed by numerous 

 and thick veins of graphic granite in which wolt'ram oc- 

 curs. 



The district of Assynt, forming the western part of 

 Suthcrlandshire, is the subject of the next article. The 

 mountains and higher ground of this district consist of the 

 same rock as ihe so called granular quartz of Jura, forming 

 here, as in the last- mentioned island, smooth conical hills 

 of considerable tic/ation, snow white at their summits, and 

 A a 2 singularly 



