THE NATURAL HISTORY OF IGNEOUS 

 ROCKS: I. THEIR GEOGRAPHICAL AND 

 CHRONOLOGICAL DISTRIBUTION. 



INTENDING in this and succeeding papers to glance 

 at some of the modern aspects of petrology in relation 

 to igneous rocks, we shall begin by considering briefly the 

 distribution of the rocks in time and space. In studying 

 the geology of particular regions this subject has often been 

 passed over, ev^en by eminent authorities, with but slight 

 notice. Indeed it must be admitted that in o-eneral io^neous 

 rocks are accorded a rather unceremonious treatment. Too 

 often they are relegated to an appendix in a memoir or 

 monograph, as if they were without significance in the 

 geological history of the district where they occur. They 

 are indeed indicated as regards their situation on "-eolocrical 

 maps, but only with a colour denoting more or less correctly 

 their petrographical nature. Often we see, on otherwise 

 carefully drawn up maps, a common legend of " volcanic 

 rocks," applied alike to Tertiary basalts and Palaeozoic 

 rhyolites and andesites. It is curious to note that while 

 stratigraphy has, so far as the sedimentary rocks are 

 concerned, replaced the old-fashioned " geognostic " maps 

 by those expressing in the first place chronological sequence, 

 no such change has been made in the mode of representing 

 the igneous rocks. It is not a little instructive, as will be 

 found by any one who will test it, to construct maps on 

 which the igneous rocks, whether interbedded or intruded, 

 are marked in the colours adopted for the stratified forma- 

 tions of like age. The distinction between intruded and 

 interbedded, and, if desired, between acid and basic rocks, 

 etc., can be rendered by different tints and hachures. 



The subject is one of considerable importance to the 

 physical geographer as well as to the geologist, if indeed 

 these are to be regarded as two distinct individuals. Al- 

 though volcanoes have been studied from a very early time, 

 and in some respects with great thoroughness, some of the 



