Vol. 56.] LA SALINE (NORTHERN JERSEY). 323 



favour the supposition of a simple intrusion, since in all the altered 

 rocks quartz accompanies the change in the dolerite. The regular 

 line of division between the quartz-bearing and quartzless rocks 

 points in the same direction. Accordingly we may suppose that in 

 an early stage in the process, but after the porphyritic orthoclases 

 had formed, an acid magma succeeded in softening and melting a mass 

 of dolerite : the augite plus alumino-alkaline constituents from the 

 felspar produced a mica ; while the plagioclase was entirely dissolved. 

 Specimens may be found from both the north and south of the island 

 in which basic elements (hornblende and mica) largely predominate, 

 but which contain numerous orthoclases embedded in them. There 

 is therefore less difficulty in concluding that, after the alteration of 

 the basic rock had been effected, a process of segregation drew 

 together the porphyritic orthoclases, plagioclase, mica, and possibly 

 some hornblende. Movement of the whole mass might follow, 

 and the quartz-bearing and quartzless parts would drag along 

 together, fracturing the felspars and producing the impression, after 

 solidification, of a regular passage from one rock to another. 



J. Arthur Phillips, in his paper on * Concretionary Patches and 

 Fragments of other Rocks contained in Granite,' l defined the former 

 as being the result of an abnormal arrangement of the minerals 

 constituting the granite itself; fine-grained; containing a pro- 

 portion of triclinic felspar often greater than in the rock which 

 encloses them ; and porphyritic, such porphyritic crystals being 

 those of the surrounding rock. These characters, he considered, 

 differentiate concretions from fragments. 



The study of rocks such as those described in the preceding 

 notes suggests the possibility of another hypothesis. We see 

 that a fragment of diabase included in granite does not remain 

 necessarily of a mineralogical composition identical with that 

 which it had before the intrusion of the acid magma, but that 

 sometimes the change is such as to result in the production 

 of a type of rock bearing not the slightest resemblance to the 

 original. The addition of quartz and orthoclase, the development 

 of biotite and idiomorphic hornblende, and the not infrequent 

 introduction of porphyritic crystals from the surrounding magma, 

 completely alter the nature of the fragment. The difficulty of 

 determining satisfactorily remnants of basic plagioclase, even if such 

 exist, frequently destroys any ground on which suspicion might 

 legitimately rest. In the absence of other evidence there is, perhaps, 

 no option but to conclude that the basic patch in question is a 

 concretion or segregation produced during consolidation. 



Additional evidence, however, is available sometimes, and we are 

 enabled, by working from the complete to the incomplete, to find a 

 cause for structures the explanation of which would otherwise 

 remain inconclusive and unsatisfactory. Taking the rocks round 

 Sorel Point as a basis upon which to work, an important clue to 



1 Quart. Journ. Geol. Soc. vol. xxxvi (1880) p. 1. 



