170 W.A. Richardson—A Basic Dyke at Charnwood. 
The Marginal Features of a Basic Dyke at 
Peldar Tor, Charnwood Forest. 
By W. Aurrep Ricuarpson, M.8c., B.Sc.(Eng.), F.G.S. 
1 the most westerly working of Peldar Tor Quarry, in the north- 
west district of Charnwood Forest, a basic dyke is intruded into 
the porphyroid. Professor T. G. Bonney + briefly described the dyke 
in 1915. The intrusion shows a remarkable laminated margin, and 
the present paper is chiefly directed to a consideration of this 
feature. 
I, CHgmicaL Composition. 
The Porphyroid—tT. G. Bonney and E. Hill determined this rock 
as a dacite. In Table I are collected all available analyses, and to 
these I have added another from a specimen chosen as typical of the 
rock in the neighbourhood of the dyke. 
It will be seen that these analyses differ somewhat. This is doubt- 
less due to variation of the specimens selected as regards quartz 
phenocrysts. For, while the rock exposed in the quarries is uniform 
when considered as a whole, yet the crowding of the quartz 
phenocrysts may fluctuate in different specimens. The analysis 
No. I from the Peldar Tor Quarry probably represents a specimen 
rather poorer in quartz than the general body of the rock, and the 
analyses Nos. II to IV are more typical. 
TABLE No. 1. 
LGA NM WEE TI IV 
SiO, 61:28 65:14 67:96 66°32 
Al,Og GN eels 14°87 13°69 
Fe,03 M22, 5°37 10°60 8:54. 
MgO 4:19 2:19 1:10 2-09 
CaO 6°38 516 1:69 2°59 
NagO 0,99; 1:56 5:07) 4-50 
K,O 3°84 — 1276} wes) I) 5 
P.O; ae = tr. 0:75 
MnO ea ON tr. n.d. 
Tgnition — | 214 — 2°05 
Total 99-96 99-44 101-29 100-53 
I. “ Porphyroide,’ Peldar Tor Quarry, Whitwick (by the courtesy of 
Mr. J. H. Robinson). 
II. Do., Forest Rock Quarry, Whitwick (by the courtesy of Mr. J. W. 
Berrington). 
III. Do., Peldar Tor (T. G. Bonney, Quart. Journ. Geol. Soc., vol. xxxvill, 
1882, p. 199; analyst, H. Berry). 
IV. Do., Peldar Tor Quarry, most westerly sinking near Greenstone’ Dyke. - 
A comparison of these analyses with the mean given by Daly * 
indicates that for dacites they are high in iron and low in alkalies. 
1 Bonney, Grou. Maa., 1915, p. 548. 
2 Daly, Igneous Rocks, New York, 1914, p. 25. 
