242 ISLA. GEOLOGY. 



is actually visible in one place, it is not less than 300 

 feet. This mass then is placed below that which consti- 

 tutes the two quartz ridges of Isla, and consequently 

 under their continuations in Jura ; being thus inferior in 

 position to the whole of that island ; where the thickness 

 of the quartz rock is not less than 2000 feet. There is 

 consequently an alternation of masses of these rocks, of 

 mountainous bulk, and in the following order; clay slate, 

 quartz rock, mica slate; since this latter substance lies 

 above the quartz rock in Jura, and, as will immediately be 

 seen, is found in the same situation in Isla. 



Two other circumstances occur in the clay slate of 

 this island which are deserving of notice on account of 

 their distinctness, if not of their rarity. The first of 

 these is the alternation of fine graywacke slate with clay 

 slate, in the beds which have been just described as inferior 

 in position to the quartz rock and micaceous schist. The 

 beds of this graywacke are divided at intervals by thinner 

 strata of the clay slate ; the first being of a pale greyish 

 colour, and the last of a dark lead blue. These are 

 regularly placed in alternation through a space of an 

 hundred feet or more, and are no less easily distinguished, 

 on a fresh fracture, by their colours and textures, than 

 by the different forces with which they resist the weather ; 

 the clay slate being the most durable. There is more- 

 over a natural division in the beds, taking place at irre- 

 gular intervals, and without regard to the situation of 

 the one or the other substance. They also present a 

 circumstance which, although not uncommon, is seldom 

 so distinctly exhibited, namely, the different positions of 

 the schistose structure and that of the beds. A sketch 

 will render this and the other peculiarities of this schist 

 more intelligible than words.* Independently of the proof 



* This sketch, (Plate XXII. fig. 6.) is a drawing of the weathered edge of 

 a bed, the lines nearest the perpendicular representing the furrows formed 

 by the weather in the direction of the fissures. The clay slate escapes the 

 action of the atmosphere, and gives the fMlaoioxis appearance of a single 



