62 PROP. J. W. JUDD ON TERTIARY 



It has been usual among petrograpbers to divide the gabbros into 

 two classes, the " olivine-gabbros " and the " olivine-free gabbros." 

 But I have shown that in most cases where olivine is thought to be 

 absent from these rocks, it has really been altered into magnetite or 

 serpentine*. I believe that all the gal3bros in their unaltered con- 

 dition contained olivine, though in very varying proportions ; and 

 that in the few cases where we find a rock of this class in which 

 olivine is not represented as an original constituent it should be 

 classed with the eucrites. Such being the case, I do not think it 

 desirable to perpetuate the name of " olivine-gabbros." 



As I shall show in the sequel, the characters of the most highly 

 crystalline of the Scottish and Irish basic rocks are exactly those of 

 the rocks which have been universally classed by geologists as gab- 

 bros. Between these and the basalts, however, we find a number of 

 other types which completely bridge over the interval between them. 

 It would be a strain on the accepted definitions of these two classes 

 of rocks to include all these rocks with intermediate types of struc- 

 ture either in the one or the other group. Consequently I have 

 employed for them Haiiy's name of " dolerite." 



In using this term I have to admit the impossibility of suggesting 

 any precise definition of it . B ut this follows as a natural consequence — 

 and constitutes, indeed, the strongest confirmation of the conclusion 

 upon which I have alwa}'s so strongly insisted — that when the micro- 

 scopic textures of the rocks are carefully studied, the gabbros are 

 found graduating insensibly into dolerites, the dolerites into basalts, 

 and the basalts into taehylytes. 



As a matter of convenience it may, perhaps, be desirable in practice 

 to apply the term basalt to such rocks only as contain some remains 

 of a vitreous base or ground-mass, and the term gabbro to those 

 which present the most distinct " granitic " structure, the felspar 

 crystallizing in broad plates, the spaces between which are filled by 

 the crystalline substance of the pyroxene and the olivine. 



Eestricted in this way, the term dolerite would be given to those 

 holocrystalline basic rocks in which the felspar appears in section 

 as an entangled mass of lath-shaped crystals, while the augite and 

 olivine occur either in definite crystals or rounded grains. 



The gabbros never occur as lava-streams. Some of the very mas- 

 sive lava- currents of Scotland, Ireland, and Iceland, however, are 

 true dolerites, according to the above definition ; though most of the 

 lavas contain more or less of a glassy base, and so fall into the class 

 of basalts. The largest intrusive masses are gabbros, those of inter- 

 mediate size dolerites, and the smallest ones basalts. In a large 

 eruptive mass the central part may be a gabbro, passing into a 

 dolerite in its peripheral portions ; it may give off apophyses which 

 have consolidated as basalt, and these latter have occasionally mar- 

 ginal selvages of tachylyte. 



* Quart, Journ. Geol. Soc. vol. xh. (1885), p. 410. 



