210 JURA. GEOLOGY. 



and are only entitled to the name of general because 

 found in many parts of the globe under similar relations 

 to the neighbouring rocks and to the crust of the earth. 

 The variations in the composition of this rock are 

 considerable, but these are generally either conglomerates, 

 or transitions to micaceous schist and to graywacke. But 

 the simpler and purer varieties predominate, and Jura 

 will therefore be found to afford very extensive examples 

 of quartz rock in its most ordinary state. The surface 

 of this rock is commonly white, but it rarely preserves 

 that colour throughout the mass ; exhibiting most com- 

 monly a grey, or yellow, or brown stain when broken. 

 The varieties in which felspar predominates, are com- 

 paratively few. Mica, as an ingredient, is equally uncom- 

 mon, and the transition to micaceous schist is rare ; except 

 on the upper surface of the total mass of beds, and 

 consequently, on the skirts of the eastern shore. Oc- 

 casionally, loose conglomerates of quartz, either simple 

 or mixed with clay, are to be seen, and these generally 

 occupy single and thin beds irregularly dispersed among 

 the rest. In some cases they contain pebbles of con- 

 siderable magnitude. In the upper parts of Ben an 

 oir, a bed occurs containing fragments of various coloured 

 quartz and of jasper; and in other situations, a con- 

 glomerate similar to that of Lunga and Scarba, is found, 

 consisting of fragments of clay slate and of micaceous 

 schist intermixed with the siliceous gravel and pebbles, 

 and cemented by the finer sand of the quartz rock. 



In approaching the eastern side of the island by 

 descending from the interior elevated ridges towards 

 the shore, the quartz rock gradually gives way to the 

 various schists, which, in a general way, may be con- 

 ceived to extend from a point near Ardfin to the shore 

 of the Coryvrechan, where they correspond with those 

 of Scarba. This line is however as irregular as it is 

 undefmable ; since the gradation resulting from the alter- 

 nation of the quartz rock with the proper schists, is so fre- 



