THE NATURAL HISTORY OF IGNEOUS ROCKS. 17 



with the Caledonian crust-movements was continued through 

 a large part of the Carboniferous period and even in some 

 districts into the Permian, while the Carboniferous and 

 Permian igneous rocks occurring in many districts from the 

 Loire Valley eastward to Saxony are clearly related to the 

 Hercynian folding. It may be noted that, having regard 

 to the petrographical characters of the rocks in question, the 

 Scottish lavas and tuffs are mainly of basaltic nature, while 

 acid types generally prevail among the volcanic rocks of the 

 same general age within the Hercynian zone ; although 

 perhaps much weight should not be allowed to such con- 

 siderations. The granites of Cornwall and Devon, it may 

 be remarked, are to be grouped not with British occurrences 

 farther north, but with those of Brittany, as members of the 

 Hercynian series. 



A point of equal importance is the frequent overlapping 

 in space of the several zones of disturbance, or the tendency 

 of a district already affected by a earlier set of movements 

 to become involved in the operations of a later set. With 

 this we may correlate the fact, emphasised by Geikie, that 

 particular districts in the British Isles have been the 

 theatre of a recrudescence of volcanic activity at several 

 distinct and sometimes widely separated epochs. Numerous 

 examples illustrating these two related principles might be 

 cited in Scotland and Wales, and equally striking instances 

 appear in other countries. Thus it is known that parts of 

 the Alpine chains coincide with older lines of folding of 

 Hercynian and perhaps greater age, and Bertrand has 

 pointed out in the Tirolese area a belt of Permian and 

 Triassic eruptive rocks branching off from the main 

 Hercynian zone to follow exactly a line laid down as that 

 of the " pre-Permian Alps" of that district. He has even 

 maintained with much plausibility a tendency for anticlinal 

 and synclinal axes of the newer folding to be not only 

 parallel to, but actually superposed upon, like axes of the 

 older folding. This hypothesis has attracted considerable 

 attention owing to its important practical consequences. 

 For instance, the search for coal in our south-eastern 

 countries will be very materially forwarded if the folds of 



