356 Miss I. H. Lowe — Igneous Rocks of AsJipriwjton. 



numerous small amygdales filled with chlorite. Similar tuffs con- 

 taining lapilli have heen collected on the western borders of the area 

 from an old quarry south of Harbertou, aud also from quarries east of 

 North Finish church and south of Diptford. 



Hock fragments are abundant in the rock exposed in the hedge 

 near Austin's Close (see Fig. 2, p. 353\ When examined micro- 

 scopically they show structure characteristic of basalts, and in some, 

 fine ophitic structure is recognizable in the groundmass. Small 

 decomposed phenoerysts of felspar, rectangular, lath - shaped or 

 rhombic iu section are abundant, and often show a roughly parallel 

 orientation indicative of flow. Amygdales are common, small, 

 irregular in outline, and mostly filled with a fine quartz mosaic. 

 Caleite patches occur, which sometimes fill the amygdales, and are 

 sometimes associated with the decomposed felspar. One of the 

 fragments contained two xenocrysts of felspar; their angles are 

 rounded, and two concentric zones of mineral development occur 

 at the margin ; an onter of felspar and actinolite and an inner rich 

 in chlorite and iron oxide granules. The felspar is decomposed 

 and contains chlorite and a lew actinolite needles. A similar large 

 felspar is found in one of the sections cut from the rock from the 

 copse south-east of Austin's Close. Numerous fine veins of quartz 

 cross the rock, and patches of quartz mosaic associated with 

 chlorite show rectangular and rhombic shape and evidently replace 

 some of the felspar phenoerysts. The silica percentage of one of the 

 fragments was determined as 596, and the specific gravity as 2'6S. 



From these facts it is clear that, although the structure of the 

 rock fragments is that characteristic of basalts, the silica percentage 

 and the specific gravity are typical of less basic rocks. It seems, 

 therefore, from the consideration of chemical composition and peno- 

 logical characters, that the fragments are of basalts which have 

 undergone subsequent silicification. In structure the fragments 

 closely resemble the unsilicified rock from the copse to the south-east 

 of Austin's Close, from which they are possibly derived. The matrix 

 of the tuff in which the fragments are imbedded consists mainly of 

 broken felspar and chloritic material and an abundance of fine 

 granular epidote. 



4. Summary. 



The foregoing description makes it clear that the rocks of the 

 Ashprin°ton area closely resemble the spilites and schalsteins of 

 Upper Devonian age in the Plymouth district, described in the 

 Plymouth and Liskeard Memoir (p. 94 et seq.), but the diabases of 

 the former differ from the spilites of the latter in having fine-grained 

 ophitic structure and in the prevalence of felspar phenoerysts. 



No example of coarser intrusive ophitic rock or proterobase such 

 as is described by Dr. Flett in the Survey memoir mentioned above 

 is met with in the Ashprington area. The sub-ophitic rock exposed 

 at Stancombe Linhay and Eaglewood quarries approaches most closely 

 to the ophitic type. North Huish and Diptford to the west of the 

 area (Ivybridge and Modbury Memoir, p. 78) are the nearest localities 

 at which coarse ophitic rock is exposed. 



