CHAP. XVII 
3/A TERIALS ERUPTED— ANDESITES 
275 
the Ayrshire coast at Cnlzean and Turnberry. Some rocks contain tlie 
vesicles only as rare individuals, others have them so crowded together as 
to form the greater part of the cubic contents of the mass. When the 
infiltration-products have weathered out, some of the amygdaloids present a 
striking resemblance to recent slaggy brown lavas ; lumps of them must 
have been originally light enough to lloat in water. 
My colleague in the Geological Survey, Mr. J. S. Grant Wilson, some 
years ago made for me a large series of determinations of the specific gravity 
of the volcanic rocks of the Lower Old Eed Sandstone of Scotland. He 
found that the andesites collected from various districts to illustrate the 
more typical varieties of rock averaged about 2 ' 66 . He also made a series 
of chemical analyses of a number of the same rocks from the Cheviot Hills, 
where they are well preserved. The results are shown in the following 
table : — 
Si02 
AloOg 
Fe.jOs 
FeO 1 MnO 
CaO 
MffO 
Na:>() 
H»0 
H2SO4 
JjOss. 
Scawd Law 
: Rennieston 
Cunrieston 
Duncan’s Dubs 
Whittoii Hill . 
Cuddies’ Tops 
Cocklawfoot . 
Morebattle 
59- 29 
62-81 
63-38 
.59-44 
60- 70 
60-58 
62-29 
59-8‘2 
16-30 
16-40 
15- 77 
16- 15 
17- 98 
12-26 
17-03 
16-96 
1-77 
-55 
-73 
1-05 
-66 
1-01 
-93 
-20 
3-70 
3- 27 
2-65 
2-83 
2-58 
4- 13 
2-44 
6-57 
-41 
•81 
-08 
-37 
-20 
-15 
-21 
-15 
4-81 
4-46 
4-44 
6- 70 
7- 07 
4-40 
3- 92 
4- 73 
3-15 
1- 64 
1-88 
2- 46 
2-20 
2-86 
2-71 
2-84 
4-19 
3-60 
1-88 
3-18 
3-57 
2-19 
1- 14 
2- 63 
3-44 
3- 02 
4 - .54 
3-70 
2- 95 
3- 61 
3-20 
3-04 
3- 84 
4- 04 
4-69 
3-35 
3-45 
-29* 
*55 
■37 
trace 
2-15 
4-81 
1-98 
* This is COi. 
The microscopic structure of the andesites of the Lower Old Led 
Sandstone has been partially investigated, especially tho.se of the Cheviot 
Hills, by Mr. Teall ^ and by l)r. Petersen,^ who both give chemical analyses 
of the rocks. Much, however, still remains to be done before our knowledge 
of tills branch of British petrography can be regarded as adequate. The 
groundmass in some of the rocks consists mainly ol a brown glass with a 
streaky structure (as iu the well-known variety of Kirk Yetliolm, and in 
the rock, still more like pitchstone, from near Airthrey Castle in the Ochil 
chain); more usually it has been devitrified more or less completely by 
the appearance of felspathic microlites, until it presents a confused felspar 
aggregate. The porphyritic felspars are often large, generally striped, but 
sometimes including crystals that show no striping. They are Irequently 
found to be full of iuclii-sions of the base, and these sometimes consist of 
glass. The ferro-niagnesian constituents are usually rather decomposed, 
being now represented by chloritic pseiidomorphs , but aiigite, and perhaps 
still more frequently enstatite, may be recognized, 01 its presence maj be 
1 V,ml. Mag. for 1883, pp. 100, 145, ‘252. , . jr-,, 
- Mikroskopische und chemisdic Uidcrswh'dwjea am Eiislatit-porphynt aus deti Chccwt mils, 
Inaug. Dissert. Kiel, 1884. Descriptions have also heeu publislied of cletaclied rocks from 
other districts, such as those by Prof. Judd and Mr. Durham of .specimens from the Eastern 
Oehils, Quart. Jourti. Ocol. Soc. vol. xlii. (1886), p. 418. 
