JURA. OEOLoar. 215 



occasions. A considerable one is to be seen iu Ben 

 an oir, which divides naturally into very small prismatic 

 concretions : in other situations they are occasionally 

 fojiated or of a schistose structure. They consist generally 

 of a dark lead blue basalt passing to greenstone. Their 

 permanence, after the surrounding strata have disappeared, 

 often produces a singular effect ; leaving them in the 

 form of irregular walls high above the level of the island, 

 and sometimes appearing, on the shores, like those in 

 Mull, to be the ruins of ancient castles. They serve 

 in such situations as registers, to a certain extent, of 

 the waste of the land. 



Fragments of porphyry are found scattered over the 

 soil in several parts of Jura, indicating the probable 

 existence of vein's of that substance, none of which 

 however fell under my notice. 



Among the beds of chlorite schist, common scaly 

 chlorite is sometimes found in nodules, a circumstance 

 not uncommon in similar situations. Occasionally it 

 is also found crystallized an occurrence much more 

 rare, and which I have not elsewhere observed except 

 in the vicinity of Glen Tilt. The crystals are of con- 

 siderable size, and present the complicated form so well 

 known to mineralogists as occurring in that chlorite 

 which accompanies the garnets of Piedmont. The quartz 

 of those veins which traverse the chlorite schist, is oc- 

 casionally found penetrated by this mineral so as to assume 

 a dark green colour ; a variety very common on the 

 shores of the Clyde in the same circumstances, and gene- 

 rally mistaken for prase. This substance occurs abundantly 

 in Borsel island, which presents a very accessible example 

 of the intermixture of the different schistose rocks ; offering 

 an epitome of the neighbouring shore. 



HAVING terminated the account of Jura, it will not 

 be useless to give a general sketch of the history of quartz 



