﻿114 



MR. G. W. TYRRELL ON THE 



[vol. lxxii,. 



Table VI. 



Si0 2 



TiO, 



A1 2 0 3 .... 

 Fe 2 0 3 



FeO 



MnO 



MgO ... 



CaO 



Na 2 0 ... 



KoO 



H 2 0 + ... 

 H«0- ... 



P 2 O s 



BaO, SrO 



F 



C0 2 



Inch ... 



44-47 

 2-73 

 7-59 

 6-25 

 9-57 



0- 4-9 

 11-93 

 10-24 



4-27 



1- 46 

 0-73 

 0'48 

 0-54 



p.n.d. 



tr. 



II. 



41-21 



3-88 

 8-43 

 5-22 

 11-54 

 005 

 11-72 

 10-10 

 5-52 



0- 03 



> 1-14 



1- 25 



0-04 



III. IV. 



42-62 

 1-58 

 6-41 

 •2-06 



11-27 

 0-03 



25 81 

 551 

 3-44 

 U-05 



0-76 



0-49 



O'Ol 



40-35 



2- 12 

 375 



3- 53 

 9-86 

 0-20 



25-69 



4- 64 

 3-14 

 0-80 



5- 28 

 0-83 

 0-25 

 006 



tr. 



VI. 



VII. 



39-93 



42-06 



40-32 



1-74 



L93 



2-66 



2-75 



12-18 



9-46 



2'49 



id-67 



4-75 



13-80 



7-89 



7-48 



004 





0-25 



32-88 



11-47 



18-12 



4-16 



11-29 



1055 



1-73 



5-10. 



2-62 



0-15 



1-07 



1-10 



0-08 



308 



0- 57 



1- 25 



0-28 



0-34 



0-68 



0-97 



Totals 



100-75 



100-13 



100-04 



100-50 



100-03 ! 100-05 100-1 



I. Picrite, transitional to tlieralite, Bellow Water, Lugar. Chemical analysis 

 by Dr. A. Scott. 



II. Auu'ite-picrite, Glenmuir Water, Lugar. Calculated from Rosiwal analysis, 

 Table V, No. I (p. 113). 



III. Olivine-picrite, Glenmuir Water, Lugar. Calculated from Rosiwal analysis,. 



Table V, No. II. _ 



IV. Hornblende-peridotite, Glenmuir Water, Luga' - . Chemical analysis by 



Dr. A. Scott. 



V. Hornblende-peridotite, Glenmuir Water, Lugar. Calculated from Rosiwal 

 analysis, Table V, No. III. 

 VI. Limburgite, Hahn, Habichtswald (Hesse-Nassau). Anal. Jannasch. Quoted 



from J. P. Iddings, 'Igneous Rocks' vol. ii (1913) p. 334. 

 VII. Nepheline-basalt, Uvalde Countv (Texas). Anal. W. F. Hillebrand. W. 

 Cross, Bull. U.S. Geol. Surv. No. 168 (1900) p. 62. 



These are typical ultrabasic rocks in their large content of 

 magnesia, lime, and ferrous iron, combined with low silica; but they 

 are characterized by comparatively high alkalies, as compared with 

 other rocks of the same category. This results from the persistence 

 of analcite into the ultrabasic end of the series, and from the 

 alkali-content of the pyroxenes and amphiboles. In this respect it 

 is difficult to find phaneric rocks to match with them. Some 

 nepheline-basalts and limburgites approach closely in chemical 

 composition (see Table VI, cols, vi and vii). 



IV. Petrology. 1 

 The differentiation of the Lugar sill may be explained in two 

 ways, according to whether it is considered as the product of a 

 single act of intrusion or of more than one. Both modes of ex- 

 planation involve perplexing features. Postulating the former, the 



1 [Since reading- this paper I have, with the permission of the Council of the 

 Geological Society, considerably revised the theoretical discussion of the Lugar 

 sill. I have done this in deference to weighty opinions expressed in the 



