F. P. Mennell — Basic Dykes and Rock. Genesis. 15 



are here well seen, as they are in a number of rocks which may 

 be described as < porphyrites ', e.g. that of Groby, in Leicestershire. 

 In the present case we have, however, clear field and microscopic 

 evidence that they represent the surviving portions of crystals which 

 have had their exteriors melted or corroded. The outgrowths are 

 therefore, as Judd correctly surmised, of the nature of secondary 

 enlargements long after the consolidation of the original nucleus. 

 Where his suggestion was at fault was in regarding the regrowth 

 as having taken place at the expense of a non-crystalline ground-mass. 

 The outgrowths that we have been discussing are clearly due to the 

 recrystallization under the influence of heat of the exterior portions of 

 the original crystals, together in some cases with materials due to 

 reactions with the surrounding crystals or with introduced substances. 

 It need hardly be pointed out that similar phenomena might naturally 

 be observed in basic dykes intruded among sedimentary rocks, 1 but 

 without the guide to their interpretation furnished by the association 

 of the acid materials with what can here be seen in the field to be 

 obvious granite xenoliths. The light such an observation throws 

 on the occurrences of quartz and micropegmatite among basic rocks, 

 and upon the frequently noted association of gabbro or dolerite with 

 granophyre, is evident. There seems little doubt that it may usually 

 be ascribed to the partial admixture, prior to intrusion, of acid and 

 basic materials, not necessarily from related magmas, or even entirely 

 of igneous origin. 



EXPLANATION OF PLATES I AND II. 



Fig. 1. Fine-grained dolerite, within a few millimetres of a granite xenolith. 

 Antelope Eoad, Ehodesia. [1737.] 



Fig. 2. Granite xenolith, completely penetrated by material from the dolerite, 

 which surrounds and corrodes every individual crystal. Antelope Eoad, 

 Ehodesia. [1738.] 



Fig. 3. Much modified dolerite from dyke near Kahlele's Old Kraal, Matopos, 

 Ehodesia. [1701.] The ground-mass contains a considerable amount of 

 micropegmatite, too fine to be distinguishable in the figure. The xenolith 

 shown consists of two individuals of oligoclase, with a peculiar cloudy mottled 

 appearance. One of these contains two corrosion cavities filled with rather 

 coarse micropegmatite, their fresh felspar extinguishing with that of the 

 xenocryst. 



Fig. 4. This shows quartz and felspar xenoliths in a slightly coarser ground-mass 

 than the previous specimen. An orthite crystal is seen as a dark patch at the 

 edge of the slice Dext to a felspar xenolith. The smaller dark flakes are biotite. 

 [1703.] 



Fig. 5. A large corroded quartz xenocryst is shown surrounded by micropegmatite. 

 The rest of the field is largely micropegmatite, too fine to show well in the 

 figure. [1704.] Nicols crossed. 



Fig. 6. This shows both quartz and felspar together with fairly coarse micro- 

 pegmatite. Note the fringe of finer micropegmatite between the cloudy felspar 

 and the adjacent quartz. [1705.] Nicols crossed. 



All the figures are magnified 20 diameters. 

 1 Compare G. W. Tyrrell, Geol. Mag., August, 1909, p. 365. 



