50 PEOr. J. W. JUDD ON TERTIARY 



types among them, and of the changes which their constituent minerals 

 have so frequently undergone, the older igneous rocks often, hut by no 

 means uniformly, present well-marked and sometimes very striking 

 differences from those of younger date. These are the facts which 

 have given rise to the opinion, now so widely accepted upon the 

 continent, that there is a fundamental distinction between the cha- 

 racters of the rocks erupted during the Tertiary periods and those 

 of rocks formed daring earlier epochs of the earth's history. 



That the differences which are ' usually relied upon for the 

 distinction between the Tertiary and older igneous rock-masses are 

 of the accidental kind to which I have been alluding, I think there 

 cannot be the smallest doubt. But, on the other hand, there are 

 strong a priori grounds for the inference that certain petrographic 

 types may be characteristic of particular geological periods ; and 

 there are not wanting positive observations in support of the same 

 conclusion. The refusal to recognize the real analogies which un- 

 doubtedly exist between the older and younger igneous rocks has not 

 been an assistance, but rather a hindrance, to the study of comparative 

 and stratigraphical petrography. The palaeontologist does not in- 

 validate the teachings of the stratigraphical geologist by admitting 

 that Ceratodus, originally found in the Trias^ is still a living genus ; 

 and the petrographer would do wisely to follow the biologist in 

 basing bis nomenclature and classification on purely morphological 

 characters, without reference to the distribution of the objects of 

 his study in space or time. If a palaeontologist were to discover a 

 fossil in Tertiary strata, presenting all the characters which he 

 accepts as distinctive of Ammonites, he would unhesitatingly refer 

 it to that group ; but many petrographers, when shown a Tertiary rock 

 presenting all the characters which are admitted to be distinctive of 

 a gabbro, steadfastly refuse to call it by that name. 



In the year 1874 I was able to demonstrate by a careful study of 

 the district of the Western Isles of Scotland, that rocks which had 

 been shown by so eminent a petrographer as Professor Zirkel to be 

 very typical " olivine-gabbros," are really of Tertiary age, and that they 

 graduate into less perfectly crystalline rocks, to which the name of 

 dolerites may be conveniently applied ; these dolerites, in turn, 

 passing insensibly into basalts, which, where suddenly cooled, as- 

 sume the form of tachylyte or basalt-glass. A study of the acid 

 rocks of the same region showed that an equally insensible grada- 

 tion could be found from granite, through quartz-felsite and rhyo- 

 lite, into obsidian*. It was therefore insisted upon that the differences 

 between gabbro and tachj^te on the one hand, and between granite 

 and obsidian on the other, are not to be accounted for by a differ- 

 ence of geological age, but are a consequence of the diverse condi- 

 tions under which the rocks had consolidated. 



•^ Quart. Jotirn. Geol. Soc. vol. xxx. (1874), pp. 220-303. In employing the 

 names " felsite" and " Mstone " in this paper, I followed an old.English usage 

 of terms. As the rocks in qviestion are, in all their fundamental characters, iden- 

 tical with quartz-felsite ("quartz-porphyr ") and rhyolite, it would, I think, 

 be wiser in the future to employ those terms for them. These rocks, as I shall 

 hereafter show, are associated with others of less pronounced acid type. 



