i8 SCIENCE PROGRESS. 



the Mesozoic strata can be taken as an accurate guide to 

 those of the older rocks which they conceal. 



The close connection between crust-movements and 

 igneous eruptions is exemplified not only by the constant 

 association of isfneous rocks with disturbed strata, but 

 equally by the absence of such rocks from undisturbed 

 districts. As instances of this we might point to very 

 extensive tracts in Russia, the Mississippi basin, and other 

 countries ; the significance of the fact being in some cases 

 heightened by contrast with neighbouring districts, where 

 the same strata, involved in a disturbed zone, are found to 

 contain intercalated volcanic formations or to be traversed 

 by dykes or other intrusions. 



We are brought then to regard igneous rocks in general 

 as in no wise meaningless interpolations in the strati- 

 graphical sequence, but rather as closely bound up with the 

 geological history of the districts in which they occur, and 

 often in intimate relation with folds, faults, unconformities, 

 and other geological accidents. Broadly viewed, they 

 occur w^ithin large but defined regions of the globe co- 

 extensive with the areas of operation of the crust-movements 

 of which those same accidents are witnesses. Further, we 

 are often able to divide one of these great regions into 

 provinces of smaller, but still in some cases very extensive, 

 dimensions ; these provinces being also defined with re- 

 ference to the folding of the region, while their individuality 

 is often marked by peculiarities, petrographical or other- 

 wise, in which they differ from one another. The eruptions 

 in one such province are not necessarily contemporaneous 

 in a strict sense with those in another, and we shall see 

 that different stages of development may co-exist in neigh- 

 bouring provinces of one region. In one province, at least 

 in the later stages of its history, we may recognise districts 

 presenting distinctive features of their own and including 

 perhaps one or more volcanic centres. 



To illustrate what we have styled provinces of eruptive 

 rocks we may take those belonging to the Alpine system in 

 Europe. Within the zone of Alpine folding itself are four 

 well-defined provinces ; the Tyrrhenian, extending from the 



