ANCIENT VOLCANIC ROCKS. 49 



their text-books and in their terminology, and consider 

 " Carboniferous trachytes " and " Ordovician rhyolites " un- 

 pardonable solecisms. 



In this country, although it has contributed far less than 

 Germany to the enlargement of the practical knowledge of 

 igneous rocks, the early and full acceptance of Huttonian 

 doctrines has permitted of somewhat wider views on some 

 questions. Moreover the attention of petrologists has been 

 especially directed to the great groups of Palaeozoic volcanic 

 rocks so fully represented in the British Islands, and so, 

 necessarily, to the comparison of them with the Tertiary 

 lavas which fill so large a space in penological literature. 

 The result has been a revolt against the above-mentioned 

 dogma and a well-founded conviction among English 

 students that the supposed differences between the older 

 and the younger volcanic rocks reduce to the fact that the 

 former are, as a rule, more affected than the latter by the 

 changes which come with lapse of time. 



Some part of the difficulty seems to have arisen from 

 confusing volcanic with plutonic rocks. The latter being 

 formed under deep-seated conditions and brought to light 

 only by long-continued erosion, those actually seen belong 

 for the most part to pre-Tertiary times ; and indeed the 

 geologists who still cling in some degree to Wernerian ideas 

 have only reluctantly come to admit the existence of 

 granites, gabbros, etc., of Tertiary age. Among the older 

 strata it is not always easy to distinguish, by field-evidence 

 alone, between intruded and contemporaneous igneous rocks. 

 Hence, when it is stated by some writers that certain 

 minerals, such as muscovite, microcline, rutile, tourmaline, 

 and topaz, are found in the older but not in the younger 

 rocks, a more correct form of statement would be that these 

 minerals are characteristic of plutonic rather than volcanic 

 rock-types. In any form the statement is not true without 

 qualification. Hypersthene is another mineral which was 

 once considered to be characteristic of the older rocks, it 

 being known at that time as a rock-constituent only in the 

 large and usually " schillerised " crystals in which it occurs 

 in hypersthenites and gabbros. Whitman Cross in 1883 



4 



